I hate Xander mostly because he reminds me a lot of people I know in real life, people who are priveledged and treat people around them like shit and never admits their short comings. This is especially true in his interactions with Cordelia and Anya. I've seen friends of mine date guys like that, and it's just as disgusting in real life, if not more.

But I think what I hate about him most is that the show never seems to realize just how disgusting he is. I can find a redeeming quality in almost any other character in the series, but not in him.