Charactor developement ! The way a part is portrayed and then "molded" or "morphed" into something different, is awesome to me ! For example: Fred, went from a very shy, innocent, Texan girl,
to a fighter & part of the team, then "Illyria" ! Wesley was a dopey, clumpsy, ex-watcher, then a fighter & brains of the team, then became a betrayer, and a very dark, dark character. Cordy went through many changes too ! And Gunn, from a ghetto street fighter, to the team, to a genius lawyer. You get my drift ?

Long-form storytelling. Seeds are planted in Season 1 (the Shanshu Prophecy, the resurrection of Darla) that continue to have repercussions all the way into Season 5. Whereas Buffy was a lot more episode-oriented, Angel is more like a novel. Especially in Seasons 3 and 4, where you really have to watch from the start of the season to understand everything that's going on.

I figured I'd keep one reason for myself, but it's true ! The writing is great, the way the whole series is strung together.
I wonder how far ahead the story line was thought out ? Only a few classic series, like M*A*S*H can be so good on an episode to episode basis. Thank you Whedon & Greenwalt !

Angel carries on the Buffy tradition of tackling realistic topics through metaphor. But whereas the younger cast of Buffy dealt with metaphorical problems of adolescence abd young adulthood, Angel ventures into the world of adult problems. The way Angel's need to drink blood while still maintaining his commitment to protect the innocent is almost analagous to a recovering alcoholic.
Unlike throughout most of Buffy's run, the main characters of Angel actually have to work to support themselves. And it's no coincidence that while Buffy never tackled the topic of pregnancy and parenthood, Angel faces it head on more than once. In the end, Angel becomes a father himself and forms a sort of family with the unlikely group of fighters he surrounds himself with.