With that said, I would be massively disappointed in Smogon if Keldeo was banned or even considered for one. If he had the Speed of Thundurus, then yeah he'd be ban-able. But no, he has base 108 Speed. There's a lot of Psychic-types hovering right above him (Lati@s, Espeon, Alakazam, Starmie) and a LOT of Choice Scarf Pokemon that can easily pick-off a weakened Keldeo (Terrakion says "Sup, little bro?") and that's on top of common counters like Dragonite, Celebi, Gyarados, & Jellicent.
It's extremely comparable to Nasty Plot Infernape of DDP OU. His Focus Blasts tear Blissey a new asshole just like Keldeo's Secret Sword does, but at the end of the day, his counters are just sooo damn common that you're only truly effective about 50% of the time. In fact Keldeo shares many of the same counters with Infernape. Keldeo's a great Pokemon, don't get me wrong #14 in DW OU usage is nothing to scoff at, but he has that exact problem of his counters being too common & his Hidden Power can't cover all of them.