Most of the ones I really hate are games where I can't join in the fun because I compare them to the games that they're inspired by, or because of their failed potential. E.g. I found KoToR (the first one, the 2nd was awesome but unfinished) disappointing because every Bioware crpg prior to that had allowed 6-person parties, and coming from BG2 with about 20 classes, subclasses and kits, and about 15 joinable companions, with different strongholds for each class, factions who you help coming to help you later in the game etc, KoTOR just felt narrow. 3 party members seemed a laughable step down from 6. The handful of railroaded joinable party members was pathetic compared to the vast customisability available in prior Bioware games. And most disapointingly, the exploration was gone - it was the advent of the Bioware '4 discrete hubs, a prologue and an ending' setup that they used to death since then (I've heard they finally moved on from that in ME2). The hub system had merits, but it meant the game was incredibly predictable - you'd know the game's exact structure ahead of time, decisions from one hub never affected other hubs and it killed the exploration factor, seeming like a concession to the handful of players who felt lost after exiting the starter dungeon in BG2 and being so overloaded with content and possible directions to go explore that they needed more handholding (which didn't seem to be a problem at the time).
Similarly, Deus Ex spoiled pretty much every shooter/rpg ever since. It just felt odd in ME not being able to use stealth and a proper sniping mechanism. And whoever it was that decided that all tech skills must become minigames should be shot. There's one 'minigame' I've ever enjoyed with tech skills and it was so awesome I can't understand why it was abandoned. I'm talking about the minigame in Deus Ex and earlier games, where picking locks/hacking computers etc would be automated, but the game wouldn't freeze and so skill level would determine (a) whether you could get in at all, and (b) how LONG it took to pick or hack. The 'minigame' was that you had to find a way of either distracting the guard or picking the lock while the guard wasn't watching or was patrolling elsewhere. Unlike genuine 'minigames' it kept you immersed in the game itself, rather than suddenly finding yourself playing frogger while wondering why the guard isn't shooting you in the back.
Never liked Halo due to lack of enemy variety. I can see why it was big on consoles though, there wasn't a lot of FPS with decent AI on consoles before then.
FO3 lost my attention after a few hours, despite my loving the 1st 2 games (I played FO2 back in the day and liked it, played FO1 a year ago and loved it, much more than FO2 actually). I could deal with the change to 3D (though a lot more use could have been made of the 3D) and the VATS/FPS, but the writing was atrocious for a series that depended so heavily on the writing. 3Dog took the cake for the: 3Dog: 'I fight using my voice' Player: [intelligence option] 'you mean you use your words to fight the good fight?' Because of the writing, the conversion of pragmatic (and hence interesting) BoS into 'save the people' knights in power armour (yes, I'm aware the true BoS appear as the Outcasts, but you don't have much interaction with them), and the cliched villains and heros, it just lost a lot of the black humour and cynicism I was hoping for.
My biggest one, though, was Oblivion. I loved Dagerfall and seriously respected and enjoyed Morrowind (aside from the game being too breakable by enchants). I was expecting a great game from Oblivion - the depth, lore, detail and ambigious pseudo-heros and pseudo-villains of Morrowind (if you pay attention in Morrowind you find there's good evidence that Dagoth Ur might be right and Vivec might be the wrongful tyrant). Instead I got an AI that was nothing like what was advertised in previews, a game where levelling up could make you less powerful, awful level-scaling, a failure to correct the most obvious flaw in Morrowind (dialogue), mass simplification of the books and lore, contradiction of the series' own long-running lore (for an obvious one, the region around the capital is supposed to be jungle, not bland disney-fantasy woodlands), cliched villains and heroes, and massive plotholes. Yes, I know plenty of folk loved it as the pinnacle of crpgs. I do wonder how many of those folk started their TES and crpg hobbies on Oblivion, making it their favourite because it was their first (I just can't envisage a fan of Morrowind or Daggerfall rating Oblivion higher than them). If so, then it did one thing right, which was bring new fans into the genre. It pains me, though, to see it hailed as one of the greatest crpgs ever made.