Far Cry. It's a pretty good game for its day, up until the end-of-level "boss" sections where you have to basically stand still and hold off a couple dozen enemies with rockets, gattling cannons, helicopters and boats while waiting for an unspecified length of time..... and managing, by sheer blind luck, to kill a gattling gunner in a helicopter from medium range with a submachine gun, only to find another guy behind you in a stationary gattling turret, or killing several boats firing ridiculously powerful long-range rockets, and two helicopters full of people, only to be pinned down with no ammo by one last 'copter with a gattling gun that will kill you in half a second if you expose but a finger..... yeah. It isn't going to return to my HD any time soon.
GTA4 I won't even bother trying, from what I've heard and some research I did myself I came to the conclusion that it sucked absolute balls.
Saints Row 2, the game runs choppily at a very low graphics level (despite many other games running fine at much better graphics), the whole "gangsta" thing is really pushed onto you whether you want it or not (e.g. I don't, I have a deep-seated loathing for the whole gangsta thing, I don't even care about San Andreas until after you get out of Los Santos, after which it rocks), and the whole respect thing..... yeah, nice idea, but not really my cup of tea.
Deus Ex. I mean, I'm sure it's good and all, and a landmark milestone game or whatever, but like someone else said before me, finally managing to kill all baddies in sight, achieve your objectives and then die because some dickwad shot you with a poisoned dart.....
Dawn of War: Soulstorm. Two words: Memory leak. You don't want to know.
Company of Heroes/Blitzkrieg II: Both fairly average WWII RTSes, Blitzkrieg II just plain sucked, CoH was more interesting but ultimately I far, far prefer the Codename: Panzers games.
X3: Reunion, I'd heard good things about the game but when I'd installed it, the lame backstory and absolutely atrocious animation in the intro disgusted me so much I never even played it.