I've personally been a fan of the direction Essentials took 4th edition, so structurally, I'm not too concerned. However, there is one thing that will inevitably suck:
A complete lack of content at first. After all, it took quite a long time to get the monk into standard play. The amount of content actually available not only shapes play experiences, it shapes campaign design. This is probably just a necessary evil of starting a new edition, but it still sucks balls.