As a raid leader, I'd love to have something similar to Counter Striker's demo system. For those of you unfamiliar, it essentially let you record a CS match and rewatch it at a later date from the perspective of any player in the match, as well as a free roaming perspective. You could control it just like you would a video (play, pause, fast forward, rewind, etc.). It would be incredibly useful to analyze what's going wrong in a given fight.
I don't believe there is anything identical to the CS replay system (it was built into the game, IIRC), but I'd love to hear about anything that might be utilized in a similar fashion. The primary things I am interested in are the positioning of players/NPCs and the actions they take (spells/abillities/etc) throughout the course of an encounter.