RTS Games in general - I used to love them as a kid, and even enjoyed some of the worst ones, but I simply can't get into them any more. I suppose, it was because I was ignorant to the various strategies, and because I only ever played CPU skirmishes, I could play the game pretty casually. Nowadays, I play Starcraft 2 online, and I'm just steamrolled before I have a clue what I'm doing. I just don't have the patience to look up strategies (or form my own), and CPU skirmishes just aren't much fun anymore.
Left 4 Dead Series - I so wanted to like these games. Hell, I feel stupid for buying L4D2 because I wasn't enthralled by the first one. They're fun going through the maps the first few times, but it becomes too damn repetitive. L4D also falls in the group of games that if you're a noob, that you're really going to bring your team down, so the community can be hostile at times. I originally played L4D with friends, but I'm not nearly as competitive of a gamer as they are, so I stopped playing with them.
Counter-Strike - It's been sitting in my steam library for years, and I don't think I've put more than 5 hours into it. I recently gave it a go, but it took me nearly forever to get in a server because it wants you to download so much shit (Garry's Mod is just as bad if you go online... thankfully that game is still fun offline), and by the time I finally got in, I lost interest. But it goes further than that. It's a steep difficulty curve, and as a noob, I don't find it fun. I've long accepted the fact that I'd rather play Battlefield or COD instead.
A couple of months ago, HL2 would've definitely been in my list, as well as Fallout 3. I got over Fallout 3 by playing New Vegas instead. New Vegas is far superior to Fallout 3, but I learned to appreciate the mechanics of those Fallout games, and I was able to go back to F3 and finally enjoy it.
HL2 was one of those games that I started many times over the years and gave up at various random points. It became more of a chore sometimes to play that game. Yea it has its weak points, but it really is a well crafted FPS. The story is definitely there, it's just not necessarily directly told to you. Once I realized that, I loved the game, and I'm eagerly anticipating HL3 and/or Ep3.