Replay mode would be a method of spectating matches of higher caliber (Tournaments) as well as matches that you participate in.
The system was put to great use in Guild Wars, to watch/spectate matches of high-end GvG, Hall of Heroes, etc. matches.
But an addition to the system would also allow replays of matches you had participated in, to view your teammates and enemies builds etc. after the match for review.
http://wiki.guildwars.com/wiki/Observer_mode
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