yeah that feature was cool. Why did they take it out? And RAGE mode in Mk vs. DCU doesn't count

Why not? For all intents and purposes it's the same thing. Unless you just really want that bar at he bottom of the screen telling you what mode you're in.

What did the aggressor actually do?
I remember it being there but never thinking "I've got to get the aggressor!"

As best as I recall, the aggressor mode made your fighter a bit stronger and faster for a short duration of time. Typically the fastest way to fill the agg. bar was to block a lot of attacks.