The most immediate change is that Mega Rayquaza is banned from ubers with immediate effect. As a result, since Smogon wants all Pokémon, even Mega Rayquaza, to be usable somewhere, a new metagame called "Anything Goes" (AG) was created, which allows Mega Rayquaza, OHKO moves, Moody, evasion, sleep spam, you name it. The only rule that applies is Endless Battle Clause. A few policies are going to be put into effect to determine whether something deserves to be shifted into AG, but nothing more than the few things mentioned above will be exclusive to it for the time being. For example, Xerneas is explicitly mentioned as something that's not even close to meeting the criteria for being banned. It's not in a different power bracket from other ubers, it has the standard BST, but an extremely useful typing and the most brutal setup move. One thing I'm REALLY looking forward to seeing is whether Darkrai will be insanely dominant in AG with Sleep Clause gone, and whether Articuno can do anything with Mind Reader + Sheer Cold allowed.
It's important to note that the line between ubers and OU will NOT be determined by usage, like the other tier boundaries. So an uber that sees less than 3.41% usage but breaks OU will STILL be considered uber, instead of some transition tier like BL, BL2, BL3, etc.
Also, Mega Salamence was quickbanned from OU, for very obvious reasons. It has unreasonable bulk allowing it to set up with Dragon Dance with impunity, and then Aerilate Return/Frustration spam just became stupidly powerful. Add Salamence's excellent movepool for taking out those things that can survive the Return/Frustration onslaught, and you have a metagame where Porygon2 is virtually the only Pokémon that can switch into it easily, and every team has to run obscure crap like said Porygon2, HP Ice Rotom-W, or Cresselia. So the outcome was fairly obvious.