Cameron has some things to say about Aggro Radii over on his blog.

The idea of the aggro radius is inherently understood by longtime MMOG fans, even if you don’t describe it as such. It’s the knowledge you develop over time, by the harsh experience of repeatedly being attacked, that there’s a magic circle around your enemies which if entered will cause them to start angrily hurling everything they have at you. Eventually, you learn how big it is and how to avoid it…

This is how the aggro radius works. Aren’t pictures great?This concept, however, is completely unintuitive for people who don’t play MMOGs. When my fiancee was first learning how to play WoW, I explained the concept to her. She hated it. Even though she knew there were these invisible circles around her enemies, she could never figure out how big they were or whether she was successfully avoiding them (she wasn’t– leveling with her was interesting in those days). It really annoyed her that she could be standing in plain sight of an enemy whose friends she had just killed, but the enemy wouldn’t even glance at her until she wandered into its circle.

I’ve mentioned before that it’s a myth that players think that they want realistic AI, but they really don’t. The aggro circle is one clear example of this. It would be trivially easy to make it so that every monster within eyesight attacked you when they saw you walk by or attack their friend. They don’t because the end result would either be an incredibly lethal world, or an incredibly huge one. The former is one that is frustrating, especially for starting players, and the latter is tedious to travel. Don’t believe me? Go fight the murlocs in WoW’s Westfall zone – they have a slightly larger aggro radius than normal, and it makes them incredibly tedious and lethal to fight against.

Aggro radii also have another good side effect, in that they allow players to actually dissect the fight before engaging, meaning that players can plan, strategize, mark targets and communicate tactics to groupmates. Raid fights in WoW where such marks happen around a blind turn are also incredibly frustrating (there are several pulls in Karazhan which are examples of this).

AI is not meant to be realistic – or at least, the realism of it is a secondary concern. The primary concern of AI in MMOs is to be deterministic, interesting and fun. Realism is cool if it happens on the way. That being said, the designers owe it to communicate changes in how things work from the real world to the players in some way.

All that being said, one meme that I wouldn’t mind seeing die is the boss creature with a room full of trash at his feet that he just passively watches you kill. Somehow, I’m not bothered at all by mindless grunts being unwilling to help their friends across the room, but its always seen wierd that Kael’thas, master of all he surveys, doesn’t feel inclined to get involved.