Sometimes when playing a game I enter a new scene and am stunned by the combination of graphics quality, motion and sounds in the scene that combine to make you feel part of the scene. At the risk of over-analyzing, here are some of the techniques that I've noticed that can contribute to this feeling:

-sound of wind blowing through trees
-trees in foreground and background having detailed movement at different speeds (e.g., outside Grayhill House in Graymatter)
-birds flying with realistic movement
-slow, realistic movement of clouds
-random splashes from raindrops (at a certain gate in Black Mirror 2)
-realistic lighting: sunset, gas light, lightning (again, BM2), candlelight.
-movement of waves on bodies of water, from swells (Longest Journey) to ripples.

I imagine some of these effects take a lot of processing power...I wonder which effects can give maximum effect for the minimum amount of computer processing? Does anyone have other favorite effects?

That's a good list!

In a city, random vehicles or people walking by.

Ambient sound that changes as you walk around an area.

Reflections from water or mirrors or shiny floors.

Which effects give maximum effect per processing power is an interesting question. I have no idea, but I'd be interested to hear if anyone else knows.

The sound of your character walking- and especially when the footsteps change sounds according to the surface you are walking on.

shadows that change according to the light source

How about -
A creaking floorboard or door as you enter a room.
A whispering voice in the mist.
A sudden loud noise that startles you.

I have a fondness for leaves blowing on the ground. I don't like scurrying insects or critters.I aslo like when you just see glowing eyeballs in the dark places.

thracia

Thunder, and rain....My favorites

I think sound has the most effect. It can bring a room to life. Imagine if everything went dead silent right now. No hum of your computer. Maybe the ticking of a clock. The passing vehicles outside.

but sounds, moving clouds, and animation would be lower processing power.
pretty much anything with water is more processing. Well depends I've seen some cheap techniques for water in adventure games.

I agree sound is a big part of the immersion. When solving puzzles where you dial a combination, flip switches, push buttons, turn dials, etc. the clicking sounds are usually spot on. The footstep effects that Sassafras mentioned are very effective: I haven't played the Syberia games for a few years, but whenever I recall the games the sound of Kate walking from gravel to heavy steel steps comes to mind. These types of sounds probably link the visual and audible cues together and make a stronger effect?

Some more effects:
-steam, smoke, fog and mist
-strong use of perspective, for example in Keepsake,
-using a necessary route with a few turns many times...after a while you have a strong sense of being oriented, as if you were there.

Hi, ergggo.

I like the way colorization of some games add to gameplay, like in The Lost Crown. Black and white at first, then slowly a splash of color, then more. Not power intensive, but effective.

Possibility does it for me. I mean, the feeling that the story is possible: it could really happen.

E.g. Zak McKracken starts in an ordinary bedroom with an ordinary guy in an ordinary city - that could be me! That could really happen!

But games that start on alien worlds or where you play as a wizard or detective, well that isn't going to happen to me in real life. So there is a limit to how immersed I can be. But Zak? It makes me want to go to San Francisco and switch on the TV - Now THAT is immersive - I already exist in that world, I just need to grab my cashcard and take a plane journey!