The idea is to encourage more players to roll their tanks out. The reward in the lfg did work but nowhere near enough of what they have thought it would.
So basically they went ahead and implemented what the community was asking from the start. They put a nice dose of cheesing into the whole aggro thingy.

As a tank what changed is that before i had to tab through a pack of mobs, now days i don't.
As a healer most of the times i heal only the tank, before was the tank and a couple of aggro friendly specs.

For a casual player this change is huge and imo they did well.