Wow, mechanics-wise, this is pretty big news. I was a bit concerned that you'd have to 'waste' a turn to Mega-Evolve (which, compared with just using a boosting move + Life Orb, would have made Mega Evolutions kind of bad for the most part). However, now that we know you can Mega Evolve AND use an attack in the same turn, it's actually amazing - as others have said, this mechanic allows Mawile to both abuse Intimidate, and then Mega-Evolve to get Huge Power, while landing a big hit in the same turn. The only downside I see is that Mawile can't use it's dual Fairy typing straight from the get-go to absorb Dragon-type moves. But then again, if the MegaEvos stick even after you've switched, then this is only a one-time concern.
Also, the confirmation of Fire resisting Fairy is kind of interesting. There aren't heaps of defensive fire-types in OU for which this really matters - but oh man, Heatran looks like it's still going to be pretty great next gen; and this does give Volcarona some set-up opportunities I suppose. Also, in Ubers, this is pretty good news for Ho-Oh, as he already has great bulk + Regenerator, so getting another resistance is pretty fantastic.