The shortest answer is rain, Excadrill, Ferrothorn, Terrakion and Keldeo
As for why it happened, only gamefreak can give you a most solid answer since it is up to speculation why they released incredibly bs abilities and mons
Permanent rain + swift swim just drowns any unprepared teams. Permanent sand + Excadrill just rips apart any teams and makes it very difficult to switch the weather. Terrakion and Keldeo along with Latis dropping from ubers kicked the speed level up a few notches, to the point where even Garchomp's was only above average at best. Tentacruel becomes a nightmare with its annoying rain dish. Spinning becomes incredibly difficult. The gems also played a huge part as it basically was a pseudo z crystal since it jacks up a move's power incredibly hard
There were but they were mostly in lower tiers. For example, Venomoth and Butterfree gained quiver dance but were still unviable in ou. There's probably too many to list for the direct buffs. As for indirect buffs, The most perfect example is without a doubt swift swimmers. Now they don't have to manually set up rain only for Tyranitar to reset it as they can reset it themselves
It's not the worst generation of power creep. It's more along the lines of 'massive'. The perspective of worst probably comes from the fact that team building is restrictive and that even uu was now basically dpp ou
Not much. If anything, the biggest impact on gen six would be the non permanent weathers even by abilities
Source: False Swipe Gaming and BKC on youtube
