Cresselia....First defensive suspect??? Maybe, but honestly I don't think it's broken.
You have shit like Houndoom, Alakazam, Scyther, Absol, Mismagius, and Umbreon. Cresselia is pretty much set-up bait because all it does is sit there and wall. Thunder Wave can be bypassed by Substitute, and it's Psychics are weak. Most physical sweepers will have trouble with her, sure, but special sweepers just walk all over her. I'm interested to try a fast CM sweeper set though because it seems it would work like Mesprit just slightly weaker but also bulkier. I don't even think stall teams will have a problem with her because they frequently use Haze Milotic or can Encore with Clefable or Jumpluff (yes my stall team uses Jumpluff). Also if you carry Moonlight you're opening up yourself to Toxic.
So yeah, Cresselia is gonna cause a few changes, but broken??? I don't really think so. Only time will tell.