Well, now that I have somehow achieved OU voting reqs using a random Crustle / OTR Slowking / DD Latios team (OU suspect) and a team of six random RU / NU Pokemon picked by people in the Rarelyused channel, I suppose I am obligated to post. The laddering has not influenced me in the slightest due to the joke teams I used and the pathetic level of skill exemplified on the ladder, so my perspective on Keldeo will be influenced solely on my experiences in World Cup / Smogon Frontier, which are much more beneficial to look at than the ladder anyway. At first, I was on the fence and leaning towards banning Keldeo, but after actually thinking about it and reading some intelligent posts, I will have to say that Keldeo is not broken. In terms of my own experience playing, only one of my teams has actually had major problems in Keldeo, but that really does not mean much. Landorus-I was definitively broken since countering it was almost impossible, not to mention it had U-turn which pushed it over the edge. Keldeo, however, has counters in the metagame. Granted, the counters to Keldeo vary depending on the Hidden Power used, but Amoonguss, Jellicent, Toxicroak (if rain is up), and Slowking counter it basically 100% of the time. Not to mention, there's also Tentacruel, Latias, Gastrodon, and Celebi, which can handle most variants of Keldeo (not to mention, other Pokemon such as Rotom-W and Latios can be decent answers). Also, to reiterate Ojama's point: all of these Pokemon have some way to recover their health. This is huge, since it makes them much harder to weaken to the point where Keldeo can sweep. Of these, only the Latis and Slowking can actually be Pursuited by Tyranitar, and in the case of Latios and Slowking, its basically a 50/50 anyway. Keldeo is useful in the metagame and helps defeat a variety of threats.
These Pokemon are also good in the metagame even if Keldeo does not exist (except for maybe, Slowking); they are not some obscure counters like Cresselia was to Salamence in OU that otherwise would be completely useless. This is the main point I want to emphasize here, since all of these Pokemon are USEFUL even if Keldeo was not around. Keldeo does not force a player to run otherwise terrible Pokemon for the sake of beating it; that would make it broken. Tentacruel is rain's best Rapid Spinner, Gastrodon walls Pokemon such as Thundurus-T, Amoonguss helps defeat offense with the help of Spore, Toxicroak is a great way to beat rain, Latias, Latios, and Rotom-W are easily some of the best Pokemon in the metagame, Jellicent is easily the best spinblocker, etc. Keldeo can be beaten with Pokemon that would otherwise be good for a team anyway; it does not truly centralize anything. It is possible to counter Keldeo, its checks and counters are all good in the tier regardless, and it does NOT strain teambuilding like Landorus-I.