Honestly, when the fuck is Wobbuffet going to the Uber tier? It's fucking worse than Garchomp.
Silly rabbit, tiers are for kids.
edit: whoops, misread your question, did they actually bring it to OU? I thought it was still Uber, that's why the following might not make sense but it's still a good read which is why I'm leaving it anyway.
They were testing it again I think, last time I checked. But the reason Wobbuffet is Uber is because it's almost always guaranteed to kill one Pokemon. How?
Your opponent has an offensive attacker out, possibly locked into it due to Choice Band....
You switch in Wobberfet somehow. Let's just say the CB'er killed one of your Pokemon. Switch in Wobbuffet.
Ok, the opponent can't switch their Pokemon due to Wobbuffet's ability, Shadow Tag. They're stuck using a Physical attack (or Special, whichever), so they're forced to use it while you use Counter, killing the other Pokemon.
I like Wobbuffet a lot but in some cases it's quite broken, which is why it's in Ubers.
I do wish they'd make it OU so I'll be able to use one. I don't play Ubers.
They may do this, for a few reasons: Strong physical dark Pokemon scare WObberfet. There's a lot of things that weaken it.
But the gamebreaking point is it's the ultimate revenge killer. The only way around it is mixed attackers, or hitting it as hard as possible and praying that it faints.
If it doesn't, you can kiss your ass goodbye.