Shall a game display the health state of an enemy ? When we display it, how to ? As part of the GUI or in an immersive way ?

Well, it really depends on the game, but let's discuss this topic in the context of FPS-RPG hybrids. In RPG games, coming from a more tactical, slow pacing corner, enemy health is often displayed, atleast after selecting the enemy. On the other hand, FPS games due to more action and fast pacing, displays the enemy health seldom or is trying to hide it in a more immersive way.

But modern games are more of a mix of these two variations. Games like Fallout3/Oblivion , Witcher, Deus Ex etc. are RPGs with a strong focus on action. Still, enemy health are often only displayed as little healthbar when pointing at the enemy.

First off there's a game impact. When fighting an enemy without seeing his health in whatever form, you never know if your attacks has any effect at all or if he is almost dead.
When you don't have this information, it is really hard to make any tactical decision (switch to other damage type, retreat etc.), other then pumping as much damage as possible (=bullets in FPS) into the enemy until either he or you are finished. Is it valid to dispose enemy health indicator in a RPG or would it kill an important tactical feature ?

When you decide to indicate the enemy health state, how to ? In slower paced games you always have the option to select and anaylse an enemy, but in a faster paced games it gets really tricky. One enemy could be handled, but what about a group of enemies ? In this case a GUI solution looks like the wrong way to go. Floating health indicators above the heads of the enemies could really kill the immersion or atleast could be quite confusion when fighting a group.

It seems, that indicating the health of an enemy in an immersive way would be the best solution, but how to accomplish this without work/art explosion ?

Even when fighting mobs of enemies, it is important for the player to know the state of the one he is currentlly aiming at.

Mass Effect, for example, had a floating life bar, and it was totally okay -- the player didn't pay attention to it, as it was just a way to give information, and the brain didn't pay to the way the data was relayed, only what it meant.

I think that displaying HP has rarelly been the reason for breakin immersion. Only way I can think of to break a HP meter would be to make it all sparkly and more detailed than the whole game. Other than that, gamers got used to how information is displayed.

While you read my words, do you give thoughts to the fact that you are staring into a flat surface, which emanates different colored lights, and you are following a string of characters composed of blac pixels? Of course you don't, your brain skips that part and allows you to translate something that is incomprehensible to a dog, to something you can understand and think over. I believe that is the case with health bars and the GUI in general

Maybe in the early days people groaned about the numbers and all different things showing up on display. Now, when we are several decades into them, it no longer matters
-- We accept displaying our ammunition state, despite the fact that no normal human could count how many bullets a minigun hosed.
-- We accept that when we look around, our character is (in most cases) composed of 2 hovering hands and a gun (in the case of FPS)
-- We accept the existence of dragons, knights that can run in 2 ton armour and space travel.
-- Finally, we accept that by punching at the WSAD and moving the mouse, we are able to look around in a game world, and in most cases we aren't aware there is a keyboard.

Evolution did all the heavy lifting for you, we adapted to how games work. You can introduce new stuff -- best way to check is to create a mock up version of a playfield in Unity, implement the way you want Health to be displayed, and take a stroll in a cube enviroment. Make the enemies move and HP counters appear and dissapear at your conviniece -- then you will be able to check if your idea is good or not. Unity is made just for that (testing stuff out), and it works well in that regard

Of course, this increases the amount of art assets, especially in an RPG where there are many different enemies. But it would definitely be cool if we could loose some the numbers from RPG's.

I think it is mostly a budget/risk issue. With so many different enemies in RPGs it would kill any budget if you would go for differnent wound representations. This could have been done 20 years ago, but was avoided due to the high production costs. Art creation in current AAA titles is even more budget demanding than 20 years ago, a typical character went through a pipeline of sculpting, modelling,texturing,rigging,animation. I bet that no publisher will take the risk to make an creature variation rich rpg this way. Any indie and hobby developer can only dream of it.
So, when using integrated, more immersive visualisation of wounds in a RPG it must be done in an other way.

Mass Effect, for example, had a floating life bar, and it was totally okay -- the player didn't pay attention to it, as it was just a way to give information, and the brain didn't pay to the way the data was relayed, only what it meant.

Could you point me to some screenshot. I tried googling, but I didn't found any screenshots with floating life bars.

While you read my words, do you give thoughts to the fact that you are staring into a flat surface, which emanates different colored lights, and you are following a string of characters composed of blac pixels? Of course you don't, your brain skips that part and allows you to translate something that is incomprehensible to a dog, to something you can understand and think over. I believe that is the case with health bars and the GUI in general

Don't missunderstood me, I'm not against GUIs in RPGs. I just want to discuss a good, modern, and feasable way to represent enemy health in a RPG-FPS hybrid. When fighting a crowd of lets say 5 goblins, wouldn't 5 floating health bars not be confusing ?

It also depends on how the player can interact with the game, I think. In an FPS, player skill is the only factor in keeping the player alive-- you can dodge around corners, weave to avoid fire, and manually target enemies for greater effect. In an RPG, a lot of that is abstracted away to dice rolls tied to stats and scripted actions.

The RPG style makes data really important. You see a lot of features that are basically stat tweaking, and the player needs quantified feedback to be able to make decisions on how to develop their character and approach challenges. A health meter is a very direct, easy, and cheap way to convey this information to the player in a form suited to tactical analysis.

There's a continuum between FPS and traditional RPG play, and the closer you get to FPS the less raw information the player will need. But closer to the RPG end demands that players get that information, and if it isn't represented as a meter or number it has to be expressed some other way. I can't think of an approach less abstract than the health meter that doesn't require some obvious signal of health on individual enemies, leading to the art explosion in your OP.

It seems, that indicating the health of an enemy in an immersive way would be the best solution, but how to accomplish this without work/art explosion ? [/quote] Here are some ways I've pondered over in which Health can be represented in a natural way within the FPS environment without Health Bars and a "work/art explosion".

Graphics

1. Procedurally Generated Damage Animations such as rag-dolling individual Limbs could be incorporated without requiring massive amounts of art.
2. Procedurally Generated Walking Animation such as those generated by NaturalMotion's Endorphin.
3. Procedurally Generated Damage Effects could be used to provide Visual clues Geometry Morphing, Real-time CSG, Particles (Fluid Loss), and Textures Blending.
4. Facial Expressions {Eyes, Eyebrows, Mouth}

Audio

1. Sound FX and Music are often reused and could provide Audio Clues to State of health . Screams of Agony (Roars, Growls) increase in frequency as the health deteriorates. Sigh of Relief is exclaimed as health recharges.
2. Dead man's Theme initiates when Health is critically low.
3. Sound of a Heartbeat increasing in pulse rate and volume as Enemy Health deteriorates.

Force Feedback

1. The sound of the enemy's Heartbe increases in pulse rate and volume until its rumble can be felt in the controller's Force Feedback.

Artificial Intelligence

1. Enemies Attack Speed, Accuracy, Duration, Frequency is reduced.
2. The Enemy retreats to avoid sustaining more damage, seeks out cover, or recharge station.