I actually love their AI system. In GW2 while being in low lvl zones you couldn't appreciate it much, but in that new 25-35 zone mobs were awesome. I mean, melee mobs knocked me down when I was trying to heal, ranged kited me, they could also work together like one centaur cast AoE on me and the other one immobilized me in place, or how melee mobs protect the ranged ones - it's great.
As for Aggro, every boss can has different formula. One is going for the weakest link (player with low on HP), the other one can react to CC, there's also a case where boss absolutely hates ranged players etc etc. Thanks to that, it's quite easy to create encounter harder than those you could see in mmos where you had strict HL/TNK/DPS roles.
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I loved this too. It was amazing how mobs protected eachother and used smart-cc!