After some UU testing, an UU tournament, and some discussion, some well-known GSC players have come to the agreement that Kingdra should be banned from UU and moved up to BL. It was deemed to be a too good defensive pokemon, that single-handedly shut down otherwise medium/top threats of the UU metagame such as Nidoqueen, Gyarados and Piloswine. However, the main argument to ban it was that it is very difficult to take on other that with fellow defensive Pokemon or defensive/stall tactics. Discussion can be found here. Skip to post #41.
It should be noted that Borat, the person who supplied the GSC UU tier list to PO in the first place, has since tried to take back part of that list by consistently rejecting Kingdra as too strong for UU. He also has a personal dislike for Slowbro in UU, but Slowbro is in line with the Smogon community UU tier list, and as such its UU presence is not nearly as contested. Just stick to moving Kingdra.
Thought I might just post to say this has been done since Moogle opted not to post I was doing something else at the time and I forgot (also can't be bothered bumping) ~CM
While I admit Slowbro is a very potent defensive pokemon in UU, it's very far from Kingdra's dominance. Everything outspeeds Slowbro, which makes an important difference when dealing with ground-types such as Nidoqueen or Sandslash, and also means that it could be taking two consecutinve Thunders from mixsweepers that Kingdra could easily handle when swicthing into them. In fact, Slowbro can't really deal with Thunder mixsweepers/users due to being weak and 3HKOed by them. It's also weak to Grass and Bug (Pinsir,Scyther). It just doesn't have the ability to stay in against almost everything of the metagame like Kingdra had. Sure Slowbro is really annoying with TWave (REALLY annoying), and hits pretty hard with STAB Surf and/or Psychic (318 spc comapred to Kingdra's 288), but then you are not sleep talking, and even if you talked, slowbro wouldn't be able to wall stuff the way Kingdra did at all.