In no particular order... *Not that I hate these guys, but they are more disappointing than anything*
Gambit - He is really not that bad, but his story as a mysterious loner just gets stale. They should really just kill him. Unless they do some cool shit with Rogue, she falls in this category too.
Bishop - Infinite potential that should and could of been reached in Messiah Complex.
Polaris - She should of been killed long ago.
Colossus - Anybody else feel he was just lame since they brought him back to life?
Marvel Girl - She just sucks. Why is she still around, and who gives a fuck?