Mobility & control. Some classes got more of it, which was not necessary.
The problem arises when you give classes strong mobility on there own, then when backed by a healer they become un-peelable (warriors & feral druids are on the extreme).
This creates a one-sided imbalance, in which casters are basically forced to tank because all of a casters snares & roots are completely dispellable, which the same cannot be made for melee snares.
the "cause & effect" of this arms race is no more evident than in frost mages, who no longer bother to sit and cast abilities (what a caster is suppose to do), its all instant icelance spam & wait for deepfreeze.