More bulk than Machamp, 129 Attack AND SpA with 252 EVs in both, better offensive typing and the same Speed as Infernape. When we first became aware of Keldeo with BW's release, I don't think I'm the only one who thought it would be ridiculous. However, that was coming from the 4th gen mentality-- people still relied on Skarm-Bliss, Ferrothorn and Jellicent hadn't proven their worth, water immunities were still scarce, and we hadn't allowed Lati@s in the metagame in ages. At the time, DPPt's defensive Pokemon had a hard time handling Infernape, but honestly-- I think BW's metagame handles Keldeo better than DPPt's handled Salamence.
Keldeo has everything in the world going for it except coverage (and having a useless ability). But that one weakness is more than enough.
I think Keldeo would be a lot harder to handle in a metagame where Physical and Special walls were more clearly divided, with Special walls weak to Fighting as was traditional in previous gens. This gen, defensive Pokemon tend to be more balanced, and it's not Blissey, but Jirachi, Celebi, or Latias that is the metagame's major special sponge. Yes Keldeo plays a role in that, but Blissey hasn't been a premier special wall for any period of this generation, and the sheer power of Focus Blast/Superpower on Alakazam, Tornadus, Thundurus-T and Hydreigon pretty much ensure that Tyranitar can't handle over half the metagame's major special threats. Unlike Tornadus-T or Landorus-I who's best checks were extremely niche (like Bronzong), Keldeo's best checks are great in number, high in reliability, and extremely main stream.
I mean, I think it goes without question that Celebi, Latias, Jellicent, Starmie, Amoongus, Venusaur, Toxicroak, etc. are all solid checks. When we're talking about base 55 and 70 moves like Icy Wind and Hidden Power as your main answers, you KNOW we're talking about relatively solid checks. I mean, in today's OU-- how many defensive Pokemon can switch in knowing the worst they have to fear is a Hidden Power? How EASY is it to take advantage of a Pokemon relying on Hidden Power, especially if it's frequently choice locked? Then there's all the other Pokemon like Rotom-W, Gyarados, Salamence, Dragonite, Ferrothorn, Gastrodon, etc. etc. who can all check it under the right conditions. This isn't a Pokemon the metagame doesn't have answers to. For a BW offensive threat, that's a lot of solid answers, even if none of them are perfect.
It has been pointed out that Pursuit is really effective with Keldeo; but it's not universal (Grass/Poisons and Toxicroak), and most of these Pokemon have ways of getting around Pursuit. Latios/Latias/Starmie can use Water attacks to outright defeat Scizor/Tyranitar that aren't specifically built for trapping them (LOTS of special bulk), and Latias can set up Reflect to get out of a pinch. Jellicent is way faster than both and has Scald and W-o-W. Celebi isn't even that afraid of Pursuit if it doesn't switch out, and even has Baton Pass-- And a team could be in deep shit if it tries to Pursuit Celebi if it has Baton Pass and has Lucario or Terrakion as teammates.
That's another thing, even if Pursuit can take down Keldeo's checks, Pursuit itself isn't a risk-less move. It's not that much use if Dugtrio comes in, kills off your Tyranitar, and now you've got to figure out how to make use of your Keldeo in the Sun for the rest of the game. It's not that great if they then set up on your Pursuit user with a Garchomg, Dragonite, or Luke and get ready to sweep through half your team.
So why are we even considering Keldeo as a suspect?
Because it's bulky, powerful, and the fastest attacker in the metagame that isn't weak to Pursuit or SR? I'd personally rather have Keldeo in that position than Garchomp-- considering Keldeo is a LOT easier to find defensive checks to.
Maybe it's because Keldeo has all those things AND hits stupidly hard with Rain boosted Surf/Hydro Pump. THAT is a pretty legitimate argument. I don't think there's any laughing in the face of those rain attacks. Except for its poor coverage movepool, this is a Pokemon really without any notable flaws.
That said though, while it is a top Pokemon, I think it is a manageable one. Certainly, more manageable than DPP Salamence (who I honestly think should have stayed OU, lol).
I don't plan to be a big advocate in Keldeo's case one way or the other, but honestly I think it falls just on the "let's keep it" side of the fence.
(No harm in testing though)