I am curious at the few posts of L4D2 myself. There aren't any huge bugs, the only real flaw I see is that Coach and Rochelle are grossly inferior to Louis and Zoey. Otherwise the only real issues are player exploits, but I will never hold idiot cheating communities against a game.
And for people mentioning Fallout 3: same solution as Oblivion, mod that baby up. The nice thing about FO3 is it doesn't run like a complete brick when you mod the game, so it's easy to fix some of the biggest mistakes of the game. Take FO3, add Mart's Mutant Mod, Advanced Cripple Effects, TAG! + Skillbook Redux, T3T Weapon Tweaks, and Repair Rethought Reborn. What do you get? An almost entirely new game. Same story, but encounters will play out much differently, with a geniue difficulty curve now. Plus if you use atmospheric stuff like Fellout MMM will make the game goddam scary. Creep around at night when it's dark as hell, suddenly a giant swarp of Night Ghouls start eyeing you for dinner and bumrush you. Chaos ensues!
Such good times. Main reason I didn't mention FO3 is compared to Oblivion, it wasn't all that problem ridden in my eyes. Needed like 6 mods to fix what Bethesda messed up, while I have something like 180 running on Oblivion and need to use Wrye Bash to make them all work. =O