Halo 2: Finished it on Easy. I think Bungie made a mistake in having the narrative switch tracks (between Master Chief and the Arbiter) every couple of chapters. It makes it significantly harder to follow the story. I could and did follow it, but it took more effort.
This sort of thing works in movies because they have more frequent switches. This prevents the viewer from forgetting what happened in one track while following another. The Empire Strikes Back and the last two movies of The Matrix trilogy exemplify this technique done well.
Games have playtimes too lengthy for this to work well. I think Bungie recognized this and corrected it in Halo 3, where the player becomes the Master Chief for the entire game.
Bungie could have made this work in Halo 2 by turning the Arbiter's chapters into short cutscenes. But that would have meant cutting the game's length by a lot, which would have not gone over well with many reviewers and players.
Duke Nukem 3D: I bought it on Xbox Live Arcade. It plays and looks just as I remember it. All the humor and gory gibbing remains intact, as do the low-resolution textures. I could wish for the high-res mod, but understand that licensing issues likely prevented them from including it.
Halo 3:: I finished it on Easy, so my comfort gaming ends - for now. For my next such experience I expect I'll play through the Half-Life series, including Portal.
Universe at War: Earth Assault: After finishing Halo 3, I started this game again. I began from the tutorial mission in order to re-learn the controls. The next night I played through the two Earth faction missions and the first Novus mission (take down a Hierarchy walker). The game seems fun, but the controls strike me as complicated and hard to remember. I may have to replay the tutorial mission once or twice to memorize them.