Like several posters have already said, I dislike Angel's habit of making extreme, life and time altering decisions for other people without their knowledge or consent. This would not be so bad in and of itself if he ever once learned from the dire consequences that sometimes arise as a result of this character flaw, but he never does. I don't think it is any small coincidence that the supporting characters on Angel end up far more dead and damaged than do the supporting characters on Buffy. Angel is a darker show at least in part because the title character is a darker character with a very different (and destructive) leadership style.
I also dislike Angel's hypocrisy. He is very harsh in his dealings with and judgments on other characters. When they fall out of line in some way, he punishes them. He tried to kill Wes and kicked him out of the group. He kicked Connor and Cordy out during an apocalypse. When he does reprehensible things, however (like impregnating Darla or letting the vamps eat the lawyers), the gang is expected to support him (and they do). At his most reprehensible, he completely disassociates, turning those actions into the actions of an entirely different person. He broods and expresses guilt, yes, but he expects everyone else to forgive him for Angelus' actions while offering the supporting characters no such leeway.
Don't get me wrong. He's an interesting character. I dislike these character traits, and yet I think they make Angel a more interesting, multi-dimensional title character than most. I like the conflict that arises largely because of these traits as they interact with his other character traits and with the personalities of those around him. I just wish that Angel had been more self-aware and maybe worked on some of these consistent flaws over the, umm, centuries.