With the zingers of dialogue, and the issues, and the compromise, and the sacrifice.
Angel needs to be renewed for a 5th season because (1) how cool is it going to be next season? and (2) lots of stuff that needs to be explained/dealt with.
Also, (1) given my personal conflict about dealing with (read: solving) world issues i adore that my show deals with moral grey and compromise and working within the system and "at what price?" and all that, and (2) how is Buffy going to top this? even with 2 episodes in which to do it.