Ok, no one seems to be talking about a major feature of mega-evolving that this Nintendo direct seems to have revealed. I may be way off base here, but the Game Freak director seemed to really emphasize forming a bond with your pokemon to get it to Mega-evolve, in fact I think he flat out stated a pokemon mega-eveolves when you have a strong bond and the appropriate item. This possibly means that you need to have max friendship with a pokemon before mega is possible, which means you cannot just trade for a pokemon that mega-evolves and then start fighting with it