Well, I don't know how it was coded, exactly, but it could be the greater % the slow is, the more agro. I'm level 65.. not low 40s, and I'm slowing at 80% sometimes and pulling agro on 300+ hitters. No one can heal me fast enough with my gear before I'll die, so I have to self heal.. which results in chain spam heals, and often death.
It's impossible to give a certain %, because the group or raids DPS will determine that, and I haven't physically clocked it, so.. No spell casting subtlety hurts, too, heh.
Please don't post that 'I am fine,' or 'I haven't noticed any changes,' just to sound tough, please.
edit: noticed that about mez, too. Encs on raids are really having a tough time, at points, but atleast someone can stand there and taunt a mezzed mob a few times before engaging.