The various Thief games had strong and negative points for each. They also had a fairly devoted following that demanded a particular type of game - a challenging game of sneaking, one that didn't have easy ways out or ways that the player could "cheat" the system (i.e. the 3rd person camera in Thief 3 that let you look around corners).

If you want to get the thief players you'll need to make a game with variable levels of skill, and variable levels of completion - it will need to be easy enough for a "normal" player to beat, but with insanely hard enough optional elements that the thief players will attempt to complete in order to score perfect. In past thief games players often made up special modes of play to increase the challenge by forcing perfect stealth - you couldn't knock out guards, except for loot everything had to be put back as it was found (even keys), it had to be like you where never there (except for the fact that you stole 100% of the loot and completed the primary and all secondary objectives). The annoying robot enemies where also a low point of Thief 2.

As far as the actual game mechanics go, you can learn from what mistakes where made in each version.

On Thief 1 Looking Glass didn't know if the stealth gameplay would catch on, so they also made the horror levels - levels where the enemies where zombies, monsters and such that where not always fooled by stealth and had to be killed.

On Thief 2, they went full into the stealth mode of gameplay, and in many ways Thief 2 was the best of the series. The only bad part was that floors where often overboard - many levels where all metals and other materials over which the player had to move at a snails pace. The type of levels players prefered where often the mansions with patches of soft carpet, where guards would follow large overlapping patrol patterns. These created the best thief experience, as the player would hide in the shadows and actually come up with their own unique plans for how to sneak past the guards and get the loot, often involving precise timing to spring out from the shadows, run along the soft carpet, and duck into another room just before the other guard came around the corner.

Thief 3 finally fixed the graphics in the series, however the gameplay was destroyed to make a console player pleasing game - even the hardest level was easy, and it was even easier if you just went about the game either running past enemies without any stealth, or just attacked and killed everyone outright. Thief 3 also seemed to lose the bow handling of the first 2 games - in Thief 1 and 2, there was a skill to using the bow (mainly in estimating the travel time of the arrow and how far it would dip based on gravity). This made it extremely rewarding when you pulled off an amazing trick shot of shooting from one raised level to another, hitting a guard just as he appeared from behind a piller, killing him with a headshot from a distance of 60 feet.

I had a long reply of comments, but my taffing Firefox browser crashed on me.

I'm a huge fan of the Thief series, and in my opinion Michalson was spot on with the flaws of the individual games in the series. The first had those annoying zombie levels, the second (although my favourite) was very hardcore (especially in the higher difficulty levels), and the third suffered greatly from consolitis.

The list of good points in Thief outweighs the bad in my mind, but since you are asking for problems I'll summarise what I had before Firefox crashed.

My main problem was the A.I. was a bit too formulaic for my tastes. I think it would be a bit more exciting if each guard reacted a little bit more randomly to various stimuli. In particular, I didn't like how the guards would overract to a single loud noise. I also thought it was a little bit odd how guards would react to any small noises that Garrett would make, but wouldn't react to the absence of noise. The interior of most levels would sound like a cacophony of footsteps, which are gradually silenced by skillful use of the blackjack. Frankly, if I were the last guard standing I would be seriously spooked out by the eerie silence of the now empty building.

Actually, that's really my only real problem, as I liked (or tolerated) pretty much everything else. I agree with the positives that everyone else listed.

Yes, the Thief series tended to combine both sneaking and a horror element - the non-sneak levels in Thief 1 could well be described as parts of a survival game (which go well with good horror theme). To be honest, I think keeping a bit of horror, in small doses, is a benefit to a thief like game. The darkness and tension created by creaping around, trying to be as alert as possible to the smallest sound, is the perfect setup to properly scare the user (for example the Cradle level in Thief 3 could scare you just from the sounds), compared to say a regular FPS, which isn't very good for horror (i.e. Doom 3 billed itself as a horror survival game, but frankly how scared can you be when your mission and "god given" abilities are to blow apart everything else in the game world).

As for AI, yes, it's not going to be an impossible challenge. The way to emulate/continue the Thief type AI is to create a simple state machine with a strong attention to the details and some descent scripting and voice acting for the various AI reactions. Essentially the AI guards should have a variable that controls awareness (Thief 1/2 had 4 levels, unaware, suspicious, searching, found), and a few other values to control things like combat - i.e. have a frustration/fear level, if the player keeps going somewhere they can't pathfind to within a reasonable time (ie they jump into a tree to escape the swordsman) or if they take too much damage, they should retreat and look for help.

The attention to detail is making the guards simple AI state affected by as many elements in the game as possible - light is out that shouldn't be out, object is out of place (weighted toward large and important items), guard not at his post - in addition to just see and hear the player. To really make the AI seem realistic, I'd give each guard a little table describing their "memory" of the world (the state of all the objects from the last time they encountered them, some weight values for how they should react). Then make it so that guards or other NPC (including non-player creatures) can "accidently" affect the world state (once you start the world going you'll need to do some major tweaking to make it not overreact and start huge chain reactions). So basically the player is hiding in the shadows a few feet from an unsuspecting guard - another guard suddenly decides he wants a snack, and changes his route to pass by the first guard on his way to the kitchen. The other guard hears this, and goes to investigate (upon finding the other guard, his "memory" of that guards position is updated, and the script "oh it was nothing" event is played back). In addition to level specific events/conversations, this makes the player feel like they are in a living, breathing world, which was a big part of the Thief series.

Append:

In light of Trapper Zoid's comments, I'd reiterate my guard "memory" device. Guards have expectations about the world (place of objects, place of guards, place of light source, place of sound effects). When that expectation is broken, it uses the weights to determine how much it has deviated to either change the guards awareness state, or simply fire an event and go on ("hmm, I could have sworn that was on, oh well"). Then once the guards awareness goes back down, the "memory" is updated - this way the guard only finds the open door odd the first time around.

Combined with the semi-random "accidental" events created by in game NPCs (guards can randomly decide to do different activies - use "The Sims" model where objects in the world advertise different types of fullfillment, so that a hungry guard will path find to a fridge, and a sleepy guard will path find to a bed) and you've created an amazingly realistic world through emergent behavoir.

I must admit I have not played alot of the thief series, but the one element I did like was haw important and imersive sound was.. it was one of the few games where I would tweek my surround system, crank the volume, and insist anyone nearby be quiet.... listening for clues, gauging how close and in what direction the gaurds were, overhearing conversations.. was a great experience...

I think "accidental" events would really go a long way to expanding on the existing prescripted conversations in Thief, plus they'd be a lot of fun - created events that can randomly occur when a certain combination of NPCs are in the same area - ie if there are at least 2 guards in proximity, there is an argument event, and a lower chance event of an actual fight (which would draw the attention of other guards). Or a guard and a dog in the same area - guard plays with dog, or dog annoys guard, or dog bites guard, or all kinds of other humorous situations (and many other combinations - guard and servant, 3 guards, peasant and town guard, etc). With only a few underlying variables and some lists you could create a very lively world (much like how The Sims works) - players might even just stand back in the shadows having fun watching the world go on around them.