The biggest thing that I hated about Spike was the treatment that he had to endure after the chip, and the fact that, because of it, he was unable to properly defend himself, especially against Riley in season 5. A lot of what Riley did to Spike towards the end was just plain bullying and frustration venting. Just once, chip/migraine and all, Spike should have decked Captain Cardboard, and I still hope to see it either in "Buffy Season 8," or in "Angel:ATF."
I know that that's not really what you meant about what I don't like about Spike, so...he should have polished his boots once in awhile. Towards the end of Season 7, they were getting kinda ragged looking. And I never really liked that black nail polish, and, just to save James the scalp pain, he should have gone back to his natural hair color after a couple of seasons. He's the only one on the show who didn't change his hair style at least once. (Well, maybe him and Giles)