I try to stay out of these threads as much as possible because I find that users make a post and then, when presented with a counterpoint, they feel obligated to continue defending their first post despite it's flaws. On IRC people tend to be more willing to change or modify their opinions, but that is neither here nor there.
Here are some loose thoughts that I have:
1. Keldeo is hard to set-up on. I can't think of anything that sets up on EB Keldeo in an effective way. Latias can spam CM while its speed gets dropped by icy wind, and it is the only pokemon that can set-up on non-choiced Keldeo. Chou so graciously pointed out why I don't have to discuss setting up on Choiced Keldeo in this post:
http://www.smogon.com/forums/thread...tial-suspect-test.3486689/page-2#post-4782436
2. Keldeo has few innate weaknesses and many innate strengths: It is not easily trapped, it resists Stealth Rocks, and no priority move does significant damage to it: At +2 Breloom and Lucario don't come close to KOing it because it has perfectly solid bulk. With it's great STAB coverage and speed, offensive teams are going to struggle to force it out, let alone KO it. Unboosted Stab outrages from Garchomp and Salamence do not kill it, nor does Terrakion's Close Combat. So three common offensive choice scarf pokemon are often unable to handle Keldeo in a satisfactory way, they'll be OHKOed while failing to kill Keldeo even after sand storm damage, so they cannot even hope to trade themselves for a KO on Keldeo. Of course I could use pokemon faster than Keldeo, but only Lati@s are good enough to fit on every team and I can probably only have 1 Lati@s on a team. A typical weatherless offensive team might have 1 choice Scarf Pokemon that can't switch into Keldeo, 1 Lati@s, and 4 pokemon that are slower or speed tie with Keldeo. I think this illustrates how much of a problem Keldeo is for offense.
3. Keldeo has counters and can be managed, but this is a result of a metagame that has adapted to it in extreme ways. To me Keldeo is what Salamence and Latias were in Dpp, and it should be banned for the same reasons that they were at the time: they centralize the metagame around certain typings. When Latias and Salamence were OU in Dpp the metagame was Dragons and Steels. Every team certainly had 2 steels, and most offenses had 2 dragons as well. Keldeo is doing the same thing now for Water. When I go to build an offensive team, I need 3 water resists because I have to deal with Rain and the possibility of Scarf Keldeo moping the floor with my frail pokemon. I've seen many players like BKC (iirc) complain that they can't use Latios because they need Latias to check Keldeo thoroughly. So it is certain that offensive teams are inhibited by Keldeo, and I doubt that anyone is foolish enough to assert that Keldeo isn't inhibiting stall: Chansey isn't being used because it can't handle Secret Sword and so stall relies on less efficient and less resilient pokemon like Amoongus and Celebi because they can deal with Keldeo. By relying on these pokemon instead of resilient Chansey, Stall becomes much weaker to certain special attackers that they could normally handle 'easily,' like Alakazam, Gengar, and Lati@s. The metagame is being defined by what Keldeo and Rain do for Water and every team style suffers for it.
I could choose to ignore the prevalence of strong water type moves and create teams that are very efficient against Sand and Weatherless, but that means essentially forfeiting matches when I have to play against Rain or Specs/Scarf Keldeo, this is because teams that are good against Sand and Weatherless usually do not contain 3 superfluous pokemon that were selected mostly because they resist water. The point I'm trying to make is that Keldeo + Rain creates a situation where I'm forced to choose between taking losses against Keldeo+Rain or taking losses against other styles because I had to dedicate so many resources in teambuilding to dealing with Keldeo+Rain.
I hope I made sense...