The biggest change for players with every expansion is undeniably getting acclimated to the new and different buttons that one has to work with, ie, the new rotation. If you poll players on their least favorite part of the next phase of the game rolling over, invariably a large percent will shout how annoyed they are with "having to relearn their class." Some abilities which were juggernauts are suddenly akin to balloons with the air rushing out of them, others that were middle of the road also-rans are suddenly hot tickets. Getting to the point where you once again have a good sense of your class is often a pain.
While the beta is far from final, and the numbers are (of course) subject to change many times over, we're still at a point where we can begin to make some sense out of the design-equivalent of watching sausage being made. We won't know be able to say with 100% conviction which fillers are best when and where, but we can assuredly say what the framework of the rotation will be.