What makes you feel like you're actually in the game world and not just playing a game? It doesn't matter how weird it is.

For me, it's the first person perspective. Dunno why, but a game tends to pull me in more if it's in first-person like Morrowind or even the old Might and Magic games. Also, realistic weather effects and character creation (especially if most of the races featured in the game are playable, if said game has different races).

What about you guys?

Writing first.

Atmosphere second.

Sound third

Graphics fourth.

I'm about as far from a graphics whore as can possibly be. Though arguably 2 is affected by both 3 and 4 to a large extent, so it's not strictly speaking a standard list.

First person perspective is important to me as well. After that I think music is the second most important thing. It makes or breaks the atmosphere more than anything else does for me.

Music that creates a mood that goes well with what's going on on the screen.

lack of npcs walking into walls <------- ( I really hate this!)

Third-person, over-the-shoulder perspective. It's hard to get immersed with first-person perspective when you have to deal with motion/simulator sickness.

2nd would be story, 3rd graphics, and 4th cutscenes.

I feel pretty much exactly the same way. Well, maybe switch 'sound' and 'atmosphere' - I guess sound contributes a lot to atmosphere? (i.e. Penumbra and Amnesia). Anyway, there's no perfect way to break it down but I feel more or less the same.

Good characters. There's nothing that immerses me more than good written characters (voiced properly if it's the case).

I hate Generic NPC no#1, Generic Party member no#2 bullshit. When they are designed right, with a proper story behind them, with proper motivation and behavior, it's excellent. Graphics don't matter, perspective doesn't matter. It's the characters that make it a living world for me.

Not taking the f***g controls away from me for s**y scripted events immerses me the most.

Good atmosphere - especially music and sound.
Good writing and backstory.
Interactivity! I LIKE being able to make stuff rather than buy it. Ultima 7 was king for this :)

...or being descended to the waist in the ground (slightly lower for ever area change until it managed to reset itself), or the same height up into the air (slightly higher... ditto), or swimming at head height. (Or perhaps my companion is just fine and it's just me that's been exposed to a dose too many of skooma while rooting out the related problem in Riften.)

What does immerse me is beautiful landscapes, and "life"... standing on a cliff and watching the landscape stretch out before me, watching a wolf chase down a rabbit, or seeing people go about their daily tasks.

... yes, my current game of choice is Elder Scrolls 5.

Remembering the last couple of games I played for "immershun", detail degradation by distance is a no-no, it was annoying having the ability to zoom in with a sniper rifle in both Fallout 3 and New Vegas while not having the detail to see anything at that distance (and this is with every graphic setting set to max, and some above through manual editing of config files to try and have stiff fade further away, not that the latter made a difference whatsoever).

My order of importance:

1. Writing
2. Ambiance/Atmosphere
2. Voice Acting
3. Other Sound
4. Graphics

Additionally, I find it much easier to lose myself in sci-fi games.

To REALLY immerse myself into the gameworld, forgetting the real world completely, requires basically two things:

1. 3D virtual goggles (like in the old Atari Jaguar commercials and other similar)

http://www.n-sider.com/media/profiles-vb022004-virtualboyprofile09.jpg

2. Two midgets rubbing my nipples while I play.

RPG wise, a well-constructed game world, great atmosphere, and NPCs with schedules, and freedom to do what you want. First person perspective helps. Fallout: New Vegas is noteable, because I realized at times I was so immersed that I actually felt like I was sneaking carefully through the wasteland, checking by back for Legion assassins, hearing fighting in the distance between a merchants guards and bandits/powder-gangers/whatever. It gave me the feeling that the world was alive. Of couse, silly bugs break the immersion...

The Gothic games were great for this as well, despite being 3pp.

I find it odd how many people say first person perspective makes them feel more immersed. I find that I rarely get immersed in first person games and feel rather detached - I usually end up losing interest and quitting. I much prefer third person games. In fact the only fpp games I remember really feeling immersed in were Bioshock 1&2 and F:NV.

Well, although it's a little shallow, I do find that I have a much easier time getting immersed in a world if the game is in first person. It probably has to do with my strong FPS background. A good visual atmosphere and really "good sounding" sound design are really important to me as well. Things like story and interaction with NPCs are really inconsequential to me. The feeling of interacting with and exploring a world is more important.

For reference, I think the most immersive games I've played are STALKER, Unreal, Noctis, and Myst, with Painkiller being a notable mention for its amazing art style.