Okay, let's see, some people are really not going to like this...
Angel. Not on his own show, but on Buffy. Twice. Killing him at the end of Season 2 made for a great finale, and while I like what they ended up doing with him for half of the season, that wasn't entirely necessary.
Then came Amends, and... his story was over there. His story on Buffy was over, and he should've left at the end of that episode, especially with that ending (don't get me started on that). At that point, The Powers had spared him to show that they had something greater in mind for him, and that's the point at which he should've left to find that destiny. Instead, he spends the rest of the season not being critical to the plot, and barely doing anything interesting (he is interesting in Consequences (for one scene) and Enemies, but only in relation to Faith, which is very telling), until he's reached the end of his contract and his show can start (really, it feels like that's the only reason he's still around at that point).
Anyway... (I don't hate Angel, I really, really don't, and I like Buffy and Angel together for the first two seasons, but it just doesn't work in Season 3.)
I can't think of any other ones, other than characters I just don't like (Riley, Kennedy), so... yeah... that's all...