I think there is some confusion over presentation with representation:
In Doom, the world was represented in 2D -- overhead sectors -- which is just another name for a closed polygon. The world was rendered as 3D (in columns) -- each sector had a ceiling height, and floor height. It was considered 2.5D because you couldn't have a room above a room.
The original Duke Nukem was also 2.5D. While it allowed room above a room it would only render one of them in this special case. All the art assets were 2D but they used sprite-scaling for a fake perspective.
The position of the enemies were 3D: x, y, z.
NOTE: The 2.5D FPS games auto-calculated the Z height when you shot your gun -- as long as you faced your enemy it didn't matter how high or low you were, you would hit them -- you can even use switches "hidden" in the floor in Doom!
Modern platform gems such as the excellent Trine blur the lines. Technically the game is 3D because the models ARE represented in 3D, but it is still called a 2.5D game because the camera is "fixed" along a rail in front of the environment. Like you said, the player can't control the Z depth.
If a RTS has the camera a fixed height from the ground is it considered 2.5D? Again, the lines are blurry. Technically 3D, but considered 2.5D. In this case the camera can move along the XZ axis but the Y axis is fixed. (Assuming X=Right, Y=Up, Z=Out-of-screen)
Trine is a wonderful platformer/puzzle/actiony game, it's very easy to play and get into and it isn't too challenging on the normal setting, but I actually haven't beaten it yet so I can't really speak about the last few levels. And it does have coop, but it's offline only and I think you need multiple mice and keyboards or a controller.
I don't think Steam is all that hard to grasp, but if you're overseas, maybe that's the problem. Installing GMod wouldn't have anything to do with steam, besides the fact that you can get it there.
Vindictus is a free MMO. People here like it quite a bit! I tried to search for lan support topics, but no dice. Uh, also, there's a character named Lann in the game, so that came up quite a bit. :)
The Trine series is available for all computers, and PlayStation. It's made for co-op! I don't know how good the LAN play is, but they do have LAN support!
I think as you get older and have played more games, you just get more selective. After you've played so many RPGs and FPSs, they all start to feel the same. I doubt it's that your capacity to enjoy a good game has diminished, but rather your capacity to enjoy any old game has diminished.
For example, I found Dragon Age really boring. Repetitive combat, bad guys attacking the land, meh. Been there done that in a dozen games, books, and anime. But I picked up Trine the other day and was hooked in 5 minutes. Something about the music, the backgrounds, the characters - the simplicity yet beautiful execution, sucked me write back in to my platforming youth.
Keep an eye out for the gems, and cherish them when you find them.
http://trine-thegame.com/site/index.php?page=buy
I find it a little strange that they don't list a linux version here. So is this just a quick bad quality port?
IIRC some of the previous humble games also didn't list a linux version on their official page, and reading this thread it seems there IS alot of issues with the linux versions?
Almost seems like one is better off buying the windows version and using wine...