I can usually tell whether I'd like a game by just looking at videos/screenshots/reviews, but a few games have majorly fooled me and had me shaking my head in confusion:
Bioshock: Despite all the raving reviews, the graphics seemed average, the story didn't capture me at all (even though I enjoyed reading Ayn Rand novels), and the gameplay was very unimpressive. I tried on two separate occasions (a couple months apart) to get through the game, and each time got bored and completely lost interest. I just don't understand what all the hype was about.
Diablo 1/2: I generally don't like RPGs but after the absurd amount of hype and good reviews, I gave them a shot. In both games, after just 30 minutes in, I was laughing my ass off at how lame these games were. Again, the graphics were nothing special, the story was lame and the gameplay. Oh man, the gameplay was embarrassingly bad. I honestly could not believe that there was nothing more to the game than clicking your way to carpal tunnel syndrome.
Halo 3: I had never played any of the Halo games, so I was excited at the chance to try this one out when one of my housemates got an xbox 360. The critics had been in a frenzy over all 3 halo games - giving them some of the highest reviews i've ever seen - so I really expected something great. What I saw instead was... just... really... really... average. In fact, compared to some of the other shooters out on the PC and consoles around the same time - such as crysis, CoD4, GoW and many others - Halo 3 looked incredibly unimpressive. The graphics were reasonable, although I hated the ridiculously colorful and plasticky look of everything. The gameplay was incredibly standard, with nothing whatsoever to set it apart from any other game, except all the weapons sound like pee shooters. The story and enemies were laughable at best. I honestly see no redeeming value in the game at all.
I'm sure there have been others, but these are the first few to come to mind...