Star Trek: Legacy. That game just sucked. Hell, the first Starfleet Command is better.
BF2142, Orange Box, Earth 2160, Genesis Rising, Armies of Exigo, World in Conflict: all fail to run (properly) on my laptop (which is basically my PC).
Ground Control 2: GC1 was fun, the squadron-based tactics rocked and were a nice change from the usual single units, the environments looked good. GC2 was just Generic RTT #2364 with single units, lame Heroes, cluttered environments, etc.
Empire Earth II: Never before have I seen a sequel that was actually uglier than its predecessor. Also, the addition of the ridiculous territory control took the whole "classic RTS" feel away from it, and that, combined with the graphical style, makes it more suited as Rise of Nations II than EE II, and I might've been more positive towards it had they published it as RoN II.
The Settlers: Heritage of Kings. Turned The Settlers from a game emphasizing on managing a "tribe" with a simplified but still fleshed-out economic system with some combat aspects to generic medieval RTS #64 billion, the economic system is a joke, the research and weather control (srsly, wtf?) sucks, the voice acting is ridiculous, the enemies are cheesy (guys in black spiky armour wielding warhammers and battle axes? Why not just carry around a sign that says "KILL ME, I EAT KITTENS FOR BREAKFAST"?), some of the characters are so retarded (especially that mayor or whatever in one of the earlier missions, where some bishop or other turns out to have sold out to the enemy - that's the point where I got completely fed up with the game and deinstalled it) that I just want to reach into the screen, pick them up and pound them into the ground until they were a bloody pulp.
WH40K DoW: Soulstorm, the fucking memory leak makes it absolutely unplayable - it seriously takes several minutes to load the campaign screen, and by the time I'm actually starting a campaign map I could've fought one battle and be halfway through the next in Dark Crusade. I'm not going to touch that game until they fix that memory leak.