Nnexxus: Bioshock. It was a very poor shooter IMO, and the whole plasmids thingy wasn't that interesting after a few hours. I forced myself to finish the game since I had read the ending was good. The story finally took off, but not the gameplay. Good thing the game was not that long : a few more hours and I would have given up.
I was also very disappointed with Bioshock. For me, though, it wasn't so much the shooting bits I didn't like (although they weren't particularly great) but actually the plot and atmosphere. It was weird, because everyone was saying that the game was steeped in atmosphere and had a really good plot, but I didn't find the characters to be believable at all, and the level design felt like a video game, not a real place. I pushed through to the end but it didn't get any better.
Also, it was annoying how similar it was to System Shock 2. I am OK with the game not being System Shock 2, but when every single mechanic was so clearly taken straight from that game, from the hacking, to the voice logs you find lying around, to the villian who keeps talking to you, it just stressed how System Shock 2 did every one of those things way, way better.