I think the Aggro icon is ugly. I think they are more subtle ways of indicating a mob is aggro. I don't need a big blazing starburst to show me.

Why not have red text, or no indication unless you click on them.
Most NPC's pulse a glow to show they are highlighted. Why not make this pulse glow red?! Then it is a surprise till you actually manually click on the character.

It's subtle, doesn't get in the way or detract from the game. Doesn't clutter the game UI. It's like /con in FFXI but without the manual method of actually typing the command or reading the result.