I hate that they're so perfect together. Wait, that's not the point of this is it? Haha.
I hate how in every stage of their relationship, Angel wants to be the martyr. He avoids her, pushes her away, doesn't fight for her like she wants, etc. But what I hate the most is that he prolongs the inevitable. Their connection was always undeniable, and he put it off. For the "right" reasons, sure, but there was no avoiding them falling in love. I just hate that he tried.
I agree with Kean here... The season 6 separation is completely stupid. It's contrary to the rest of the series. Angel has always been there for the pivotal moments, and I HATE with a passion that he didn't make an appearance (save the off-screen one that I'm not counting) because Buffy switched networks. Nothing gets to me more then when a show has to change the plot, or characters because of a network or availablility issue. UGH