Now that OmegaRuby/AlphaSapphire has been out and we got amazing new megas (that aren't Steelix/Pidgeot/Audino):
1. Gunk Shot was a godsend to Greninja. Even with an uninvested Attack stat with a reducing nature, with Gunk Shot, it still 1HKOs Mega Gardevoir and 2HKOs other OU fairies such as Azumarill and Togekiss. That's the power of Protean on a mon with very good stats. Geez.
2. Pangoro got a lot of nifty tools. Knock Off specifically. It's quite a force to be reckoned with in the lower tiers, and might even be viable in OU.
3. Adaptability Dragalge's Draco Meteors hit harder than Latios's with a Life Orb. Yeah, the thing's a nuke. It's also even more viable than it was in the higher tiers.
4. Mega Salamence was guaranteed to be quickbanned. Amazing ability, amazing movepool which means it can run various sets, amazing bulk (especially if base form is running Intimidate), and it's overall just crazy good. It was pretty much guaranteed to get the boot.
5. Who says Mega Slowbro didn't need Shell Armor? With that in conjunction with amazing defensive bulk, Calm Mind, and/or Amnesia, you have yourself a crazy good wall. After a few setups, it becomes incredibly tough to take down.
Commence discussion.