Haven't tried L4D2. Bought FC2 and hated it
I'm not sure if you're making a comparison here, but if you are, FC2 and L4D / L4D2 are nothing alike.
L4D2 is a great co-op game with a somewhat satisfying single player aspect as well. It's nothing like BF2 or any of the "open area" FPS games, which can be a nice change. The fact that you need your entire time with you to survive, and have to go back to help someone up if they're incapacitated keeps you from simply charging ahead and gunning through everything in a frenzy. I also find that like Counter-Strike: Source, the weapons in the game feel "realistically powerful" compared to games like BF2 and COD4.
Speaking of CS: S, that game has kept me coming back for years. Even though it's also long in the tooth, the number of mods and servers with various rules can change the game play immensely without any effort on your part. For example, I've played the vanilla game (mostly cs_office) for years, and recently got bored with it. So, I started playing on a gun game server. When I got bored with that, I started playing on a mini game server (air hockey, moving train-top knife combat, jumping puzzles, custom weapon arenas, jet packs, etc.). Now I'm taking a break from that and playing a vanilla server again.
Team Fortress 2 might also be worth a look if you want a change of pace.
Finally, and oddly enough, I'd suggest looking into Fallout 3. It's an RPG, not a shooter, but it still has plenty of shooter elements. It's far from the best game ever created, but it has a decent-sized world with lots of guns, melee weapons, and things to kill. Best of all, once you're out of the starting area, you're free to go anywhere and do anything.