Halo 3's predecessors made for a pretty rough act to follow. Aside from being the first Halo game on a new generation of consoles, what could Halo offer to the series that would have the same gravity as Halo 1's general existence and Halo 2's standard-setting multiplayer? The non-ending, second game in a trilogy sort of ending that Halo 2 had didn't really leave Halo 3 much room as far as story and game universe goes; Halo 3 had to continue the saga of Master Chief, the Covenant, the Flood, the Brutes, and so on. As such, the single-player campaign for Halo 3 was left to gamers' minds as a foregone conclusion: there will be more Master Chief, the loathsome Flood would have to come back, something about Cortana, the come-uppance that the Prophet of Truth has coming, and all of those other story threads that exist within the Halo universe as established by the first two games.
And that's exactly what happened. Halo 3 is the kind of game that everyone expects to be excellent, polished, and all-around amazingly-crafted game experience. And it is. The problem with the single-player portion of the game is just that: it's as expected. The original Halo came out in 2001 and its core design principles are still heavily intertwined into every fiber of Halo 3's existence. It's a game where none of the weapons have an iron-sights aiming mode, where players can run and shoot their assault rifle without noticeably increased bullet spread over standing still or crouching, and where players have one movement speed with no spring or stamina. In some ways, Halo 3 echoes of first-person shooters of yore, which focused on action, cool weapons, and input simplicity. Halo 3's core mechanics are focused in a completely different area than so much of its competition.
What Halo 3 does is to provide a series of wide-open and interesting encounters for players to utilize every strategy, play-style, and tactic they have in order to complete the encounter. Halo 3's best "levels" are not the ones with the most awe-inspiring scripted events or action-packed shooter segments. Halo 3 is at its best when a level is composed of a series of discrete encounters that span wide, open outdoor environments (which is in line with Bungie's "30 seconds of fun" mentality). It's a game about surveillance, execution, and reaction.
When a player first enters an encounter space, the enemies are almost always unaware of his presence. If there are marine AI entities following the player, they will stand still and remain inactive until the player makes his first move. During this time, the player is free to wander around areas which are very clearly outside of the enemy's viewing range. Players can get an idea for enemy groupings and patrols, what kind of weapons they're rocking, any hidden snipers, and valuable mid-battle weapon caches that will be of use once the encounter starts. The caches are particularly of note because regardless of a player's first move: the resulting battle will never end with a swift, tactical execution of actions. Halo 3 is not a game like Rainbow Six where the best room entry is one where all enemies are neutralized simultaneously; Halo 3 is a game where players have to weigh the threat level of every enemy and attempt to take out as many high-threat targets in the first phase to ease the difficulty of the ensuing chaos.
A high-value target in the game is very dependent on circumstance. In some cases, the high-value targets will be vehicles or turrets which, if a player is on foot, are almost always more deadly than any single enemy soldier. In the absence of vehicles, the high-value targets are group leaders. Halo 3's AI is organized hierarchically so in the absence of generally dangerous vehicles roaming the battlefield, the high-value targets for players are the leaders of a group. A group of Grunts will have a Brute as a leader and while that leader is alive, the Grunts are an organized and somewhat formidable arrangement of enemies. Once the leader is dead, though, the grunts are scatter-brained, frightened, and prone to just running away and hiding. I am unsure if group AI exists within a formation of higher-level enemies like Brutes, as later in the game there is a clear "Brute Leader" in a given pack.
Once the player has his plan of attack internalized, the execution phase begins. The player's first action will be to take out the high-threat targets that he noticed during his surveillance; if he's lucky, he can get maybe a fraction of what he had intended (the reality of his plans will become more clear through trial and error). At that point, all of the enemies in the battlefield are actively engaged in combat and will act within their series of groups.
And everything after the initial attempt at execution is, essentially, the reaction phase. When every enemy is in battle, players have to constantly be reacting to the death of squad-mates (who are only sometimes useful), the movement of enemy groups throughout the entirety of the encounter landscape, and the player's own shield. A player's shield can't take much of a beating before it sends players into what feels like a near-death state when the shield bar is flashing red and the player feels like just one more hit would kill them. This is an interesting phase since, really, even up to Heroic level diffculty, players can actually take a surprising amount of punishment in the near-death phase. Halo 3 just makes that phase feel like near death to signal that players need to find cover imminently. At this point players will move from cover to cover -- as Halo 3 is a "loose cover" game unlike the "hard cover" of Gears of War or Killzone -- as they attempt to eliminate enemy by enemy on the battlefield.
Also during the reaction phase is the concern of weapon type, ammunition, and vehicle usage. One of Halo 3's most prominent and important design principles is the focus on player motivation/movement through resource scarcity. Since no single weapon ever really allows players to fully "stock up" on ammunition, every player has the constant goal of finding ammunition or new weapons to use mid-battle. This forces players to adapt a roving point of attack throughout the duration of an encounter. Aside the inherent tension and urgency this gameplay feature adds, it forces players to see an outdoor environment as more of a "level" than simply an unnecessary, although attractive, vista. And the utilization of Halo 3's incredibly fun and well-implemented assortment of vehicles lends an entirely new layer of complexity and replayability to the encounter as a whole.
The entire surveillance, execution, reaction concept is, essentially, the idea behind Far Cry 2 design Clint Hocking's intentionality and improvisation idea (presentations: Intentionality and Improvisation). Which is that a well-done game based on emergent gameplay design allows for players to spend time formulating a plan of attack and then have a given game turn that plan upside down and force the player into a quick improvisational phase (reaction) where he forms a new plan of attack based on his new situation. It's an excellent way of keeping players continually immersed in their combat experience by, essentially, tossing a wrench into the innards of what they thought was a well-laid plan. For most players, the fun of combat isn't having everything go according to plan, but rather adapting to a plan gone haywire as a result of external factors.
Halo 3, like its predecessors, breaks down when this gameplay model is violated for the sake of narrative continuity and "variety." Unlike the complex AI that governs the Covenant forces that players fight throughout most of the Halo games -- the heirarchal AI that is incredibly easy to recognize by any player due to its human-like behavior in combat -- Halo has always had "The Flood" come into the game at some point. The Flood are savage, unintelligent, and incredibly aggressive enemies that follow no real recognizable AI patterns other than: see human then attack human.
This strategy would entail its own set of player strategies and reactions if handled properly, but one of the issues with the Flood in the Halo games is that their introduction into the game world is almost always coupled with terrible, confined, indoor-heavy level design. The most egregious offense of which is the "High Charity" level in Halo 3 where players enter a Flood-invested ship from Halo 2. In this level, the entirety of the aesthetic is an orange, red, and brown-heavy color scheme coupled with thick murky atmospheric effects, and a constantly feeling of claustrophobia. This level is also incredibly confusing to navigate and results in numerous points of player confusion due to a complete dearth of recognizable interior landmarks and an overly organic architectural style which is not conducive to any player-recognizable sense of flow.
The Flood's issues run further than any given level, though. Bungie has valiantly tried three times to make this alien race more palatable to its players, but the issue each time is simple: the Flood are visibly-brainless creatures in a game which has no need for them. The foundation of one of Bungie's most talked-about design principles is the "30 Seconds of Fun." I can't find a definitive reference for this, but the gist of this principle if Bungie can make an encounter or scenario as fun as possible for thirty second bursts, then they can string together those scenarios back-to-back for an enjoyable gameplay experience. If this is the company's approach to Halo 3 -- a game which has a six-to-seven hour long campaign -- why is there the need to add an entirely different enemy type which provides for a completely different play experience more than half-way through the game?
Hypothetically, a game's campaign from a ludological perspective is the slow progression of a player's mastery of the game up through the ending, which is the culmination of all of the player's skills in some glorious ending segment. If we take this as the case for Halo 3, then the player learns the ropes of the game in the first level, runs out of new content for his primary toolbox around the half-way point, and is then required to think about everything he was taught in new, more profound ways as he is pit against increasingly difficult combinations of enemies as he nears the end-game.
The introduction of the Flood is essentially forcing a completely different style of play on Halo 3's players just as those players are interested in taking everything they have learned about the game up to that point into bigger, more dangerous battles. Instead, they are forced to play a simpler, more run-and-gun play style against a variety of enemy who are strong and stupid in some of the game's most uninteresting and traditional level designs. It's a strange, undesirable thing to force upon a player who is, at that point, feeling like they "get" the game and are looking forward to applying their mastery on an entirely new level of encounter complexity.
This is all made worse by Halo 3's treatment of the Flood being a surprisingly complex one. While the basic operations of the Flood are to attack the players with absolutely no care for their own well-being, the Flood this time around -- and I didn't play Halo 2 as much as the first or third game -- are an incredibly dynamic, ever-changing enemy force. There is one Flood enemy that, from its base form of a squirmy, crawling Spore-like creation, can turn into a turret capable of mounting on any floor, wall, or ceiling or, alternatively, can turn into a hulking beast with enormous strength that's incredibly hard to take down. And, while all this is happening, Flood spores are roving around the level looking for new bodies to infest and breathe life into that, but if the player manages to kill all of the spores then there will be less enemies to deal with. It's a completely different type of enemy than the human-like tactics of the Covenant that the player sees throughout the other 80% of the game.
It's hard to definitively say whether or not a Flood-less Halo 3 would have made the ending stretches of the game a repetitive, painful endeavor to complete. At the time of writing, I have played through the entirety of the Halo 3 campaign three or four times and I have played through the gorgeous, intuitive, and and well-paced introductory handful of levels a couple more times than that, so I know my response to that scenario. The Flood levels aside, a number of Halo 3's more wide-open levels (some from the beginning and some from the middle) have always stood out in my mind as being some of the finest examples of what an action game can be if games open up their levels and expand the capabilities of their AI a little bit.