Ok, indepth comments.
Going to OU was a bit more surprising than some other hopefuls such as Porygon2 and Umbreon, though it's not a complete shock. UU players are kinda hit because of his versitility in the tier, but "too bad." The primary reason for the increased usage is probably its ability to function well as a lead, seeing as how 42% of it's usage came from leading. Sleep + Spikes is a damn good advantage for a team. With Aerodactyl being virtually the only Taunt lead (everything else forgoes taunt for other options), it has a damn good chance of pulling off its strategy too. It doesn't even have to lead either; it can wait until a slower pokemon appears and pull the strategy from there (you'd be surprised how many pokemon don't pack much speed). Statistics don't entirely reflect my above reasoning, but I like to pretend a pokemon isn't OU because scrubs use it's "omg pwnage" sets.
Donphan: God damn it Donphan!
The drop to UU is really no surprise; there's been talks about it for awhile, and people theorymon'd it to hell (it was probably dropped in usage to spite you guys, HA). The fact that it dropped and Donphan didn't pisses me off, but I'm not going to discuss that (no seriously, how is Donphan still OU. WTMFF?). Yanmega will definitely be a key player in UU, but I have my doubts that it will be game breaking. Stealth Rock utterly destroys it, and with virtually no way to boost its special attack, it will fall short of expectations. The only problem is that if you don't get SR up, it might be a bit hard to swallow (don't expect a 6-0 against a "good" team though). Registeel and Chansey will do a good job of walling it. (Hint: Try passing Nasty Plot from Ambipom!)
: Hail is going to dominate, hard. Snover and Walrein are allowed, along with a plethora of other hail accessories. Abomasnow's move to Limbo really dropped hail usage, so NU will suffer until it's banned from NU. After hail is banned, Sandstorm will likely dominate with decent rock options available like Cradily and Relicanth. It won't have an easy time however; NU is far from the "most useless pokemon in the game." For a tier associated with Luvdisc and Farfetch'd, there are some damn decent pokemon there. There will be gimmick attempts to screw around with rain/sun themes and bad pokemon "just because it's NU," but after awhile it'll mold into a serious tier. Sorry folks, but if you thought you'd see the day where Flareon rips shit up, think again.