Remember when you summoned a infernal and sometimes you couldn't control it and it would attack everyone? Why can't we have more awesome abilities like that? Where the abilities have a huge benefit, but its a risky and may cause more trouble than good.
Would love a Pack Form for druid where you turn into a wolf for a limited time, but you have a chance to be hostile and lose control of your character. If, however, you successfully control the form you get a huge damage boost for a limited time.
( For those who don't know about the pack form - http://wowpedia.org/Pack_Form - its basically where Worgen originated. )
Or something like Arcane Bomb, where you toss a bomb of arcane which does a great amount of AoE damage but if you don't choose where you want it to go in, lets say... the next 3 seconds, It will explode on you and knock down half your life.
Or something for shaman where you call to the earth to use a powerful ability (Such as summon a storm of elements) but there is a chance you get frozen in diamond for 10 seconds, much like Muradin in Old Ironforge.
I mean in lore there are tons of abilities that are banned or are extremely risky, I'd love fun little abilities like that in WoD that bring consequences.