I completely agree with windsong about Cresselia. Cresselia is very centralizing and often requires 2 Pokemon to take it down. However, its presence in the tier makes a huge difference, but a good one from what I've seen. RU teams in this stage have so much more variety, and many users have been discovering some great underrated threads such as Pinsir. This creativity may be a direct result of Cresselia. This is because Cresselia counters so many threats that it basically gives any team an open slot. In the last metagame, for example, some teams needed 2 Pokemon to counter both Nidoqueen and Sceptile (for lack of a better example). Now, Cresselia covers both of those, meaning that there's room for another Pokemon which can fill a different role. This is exactly what's making almost every playstyle viable. I still haven't faced many Cresselia so I wonder if it's too hard to take down on full stall. You can essentially keep Cresselia alive for the whole game with enough support from teammates such as Poliwrath, and wear down opponents with hazards due to the strong core being able to force so many switches. This is just theory, though, but Cresselia can possibly be over-centralizing and thus detrimental to the metagame. I don't think that we have discovered its full potential yet.
Despite this, I do not want to go back to Stage 10 under any circumstances. That metagame was absurdly boring and not fun to play. You saw the same offensive teams everywhere, boring F/W/G cores, and nothing too creative or exciting. The cause of this, like windsong said, may have been Nidoqueen, who undoubtedly limits defensive teams. Cresselia certainly brings balance to the tier, but it may be too powerful if people start to make more full stall teams. I think that it deserves another round of testing so we have more time to build teams with Cresselia.