Jet Force Gemini - Heard lots of good things and picked it up at a car boot sale for a fiver. Just did nothing for me.
Halo series - When I got a 360 I bought a group of regular Xbox games, two of them being Halo 1 & 2. Was really looking forward to it as I had to wade through the ocean of jizz people had fired out over this game but found myself unimpressed. It was an OK shooter, but it didn't stand out as anything.
Mass Effect - Not sure what people saw in this too. Maybe I just wasn't interested to begin with. I probably will give it another chance in the future.
Super Paper Mario - Meh.
Advance Wars - I bought the original after buying Dual Strike, I got so used to using the stylus that using the D-pad was a chore.
Final Fantasy VII - Superficially the graphics turned me off this one. I know, sillies.