Mostly been listed: Oblivion, Deus Ex, Spore, X3 (X1 + 2 as well for that matter, I really
wanted to like them), DoW2 (I played 1 obsessively, I dunno what the craic with 2 is but I can't be bothered with it).
Any C&C game released after RA2 has been crap.
The Sims. Urgh.
Any "Sim-" game after Sim City 2000 has been un-engaging to me. Played SC200 to death though.
All that Call of Duty (and similar like Medal of Honour) crap. I swear I suspect more man hours have been expended by gamers playing WW2 FPS sims, than were expended by actual people fighting in WW2. Yes, you might say they've moved away from the WW2 era at bloody last, but its still the same game with a different dress on.
Didn't like Fallout 1,2 or 3 much, but loved Fallout Tactics. Some people probably want to push rusty nails into my face after reading that.
A fair few of these Indie games on Steam. If I want to play a game that looks like it belongs on a free flash games website, I'll play it on a free flash games website. Braid was pretty damn good however.
Any of those Football Manager things that look like colourful excel spreadsheets to me. I don't see the appeal at all.
GTAIV (Singleplayer). Loved the rest, but just can never be arsed finishing this one. Multiplayer is a good fun however.
Warcraft 3. Meh. Its like Starcraft. With Elves. : /
SWAT (or whatever its called these days) too much damn planning needed, why cant I just kick the doors in an shoot the terrorist in the face, and the hostages too for good measure?
Those Lord of the Rings Games. Hell I'm playing them because I thought they would be like the films. They were sadly however not like the films.

thank god it wasn't me who bought them.
Age of Empire. I just cannot play this game at all, the whole way the entire game works is pretty damn retarded in my mind. Playing it makes me want to chew off my own knuckles so I don't have to put up with interacting with the game any more to find the exit button.
Microsoft Flight Simulator. Where's my damn guns? What is the point in this game? Fly from A to B. And then? Fly back again, or something. I dunno, I love my Flight Sims like but flying Civilian things just isn't fun.
Any Final Fantasy. Just no. Doesn't interest me in the slightest. I tried, I tried so hard when people were going ape**** about FF7, and raving about their giant colourful chickens, and some Round Knight chap. But it just didnt interest me at all.
Consoles in general. Not enough keys! These days to do anything in a console game you seem to have to twiddle several bits of the controller at the same time while licking your own elbow. Why don't they just get a keyboard and mouse and be done with it?
I imagine theres loads more.
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On a lighter note one game I did buy and thought I'd hate was Indigo Prophecy. Absolutely loved it, played through the entire thing in one day, barely moving from the PC, it sucked me in from start to finish.