A lot of people seem to hold the fear that smogon flirts with being "banhappy" when it comes to its OU decisions. I actually broadly think this is a healthy mentality; only the bare minimum should be made illegal in the standard metagame. Sirlin's Playing to Win series helpfully posits that bans should only occur in the most dire of circumstances (ie. an element so powerful the game wasn't meant to handle it, such as Street Fighter's Akuma, or something which otherwise pulverizes most of its matchups with little trouble, such as Metaknight in Brawl (although even that one is iirc still hotly debated)). I think fundamentally uncompetitive elements like MGengar, multi-Sleep and OHKO moves easily qualify; BW2 Keldeo does not.
HOWEVER -- and this is something I've had to mentally adjust to with Pokemon -- when you're dealing with upwards of 800 threats, most of which are viably customizable and MANY of which are specifically designed to be incredibly unbalanced and powerful for flavor to begin with (broadly, legendaries), you're naturally going to have a disproportionate amount of banworthy components compared to other games. I believe MKang is far and away the most straightforwardly bannable pokemon to hit the scene in a long time. He simply plays like an Uber: steamrolling switch-ins with little effort or prediction, finding ample set-up opportunities with one of its coverage moves and any number of viable teammates, boasting limited and very weak counters who serve little utility outside of putting a shaky stop to this one incredibly powerful threat.
As a newer player (very long-time lurker, started playing Showdown in the last months of BW2 where I had a lot of fun and success with a braindead rain team), I honestly have trouble summoning the willpower to hit the ladder right now with MKang running rampant. I adore the Mega mechanics and all the other competitive-minded changes XY brought to OU, especially creative threats like Gale Wings Talonflame and the great Stance Changer and even the other extremely powerful Megas (Luc, Pinsir, Mawile, none of whom I think are close to bannable and I think add great flavor to the game). But MKang causes me pretty bad anxiety. It makes the games unfold in ways I feel they shouldn't -- forcing multiple carefully selected successive sacrifices for non-specialized/teched-out teams to take down -- a solid grounds for just getting rid of the damn thing so people can play the game again.
EDIT: Actually I'd like to add that I don't think Seismic Toss/Wish/Protect/Crunch with mad bulk has been getting enough love in this thread. I think the set is incredibly strong and cool, and while I don't believe it's nearly as unmanageable as PuP, a mon this good having multiple extremely viable roles is scary.