I'm not sure of how many polygons Unreal Tournament characters have, but that's low poly to me. System Shock 2 models also count, they're lower than UT I think.
Most people, especially 4chan people, would probably call those models unacceptable, but for me, gaming is about gameplay, and recognition of the models, not about detail that isn't used during gameplay. (I still find the world of Unreal amazingly beautiful, and SS2 is wonderful.) I don't think I'd call System Shock 1's environment "beautiful", but I think it's perfectly suited to the game, with the simplistic level of detail.
I think most models should stick to Unreal Tournament 2004's level of detail, or less. When stuff gets really high polygons, it can be annoying, especially if they have contrasting textures, and flicker without 8-16x antialiasing.
P.S. I just looked up UT2k4's polycount, and I guess most of the models might be 2-3k, while custom models would go up to 6k or higher. TF2 models are 4-6k which I think should generally be the highest any models should go up to, except for certain parts of the body (face/head mostly).
Here's an article on a low polygon TF2 pack, with download reference:
http://www.gamersglobal.com/news/561P.P.S. I might not be the right person to ask, as I've been playing games since before 3d consoles (too easy), and I haven't played any new games, my most recent is STALKER or Bioshock, probably. For good low-poly stuff in an RTS-way, look at the models for Earth 2150. Those are deliciously low-poly, but it fits with the style of the game.
