Mega Sceptile is exactly what I look for in a Mega; more detailed/complex features without being over-designed, complete with a new typing and unexpected ability. It's already one of my favorite Megas. There are some type liabilities, but I'm still a fan, and I'm sure it'll lessen Wash Rotom's use in the meta. Wouldn't mind it somehow getting Draco Meteor, though...I'm no expert, but I'm pretty sure it's strongest Dragon STAB right now is Dragon Pulse.
I'm slightly less thrilled about Mega Swampert (it just doesn't seem to fit with Mega Sceptile and Mega Blaziken), but it's grown on me already. I'm sure Swift Swim will come in handy... Based on its bulk, it looks like a loss of base speed is possible, but there are enough opportunities for rain in Hoenn to make up for it.
I wasn't expecting such dramatic redesigns of Team Magma/Aqua, either. Archie looks pretty similar (now much more badass), but I had no idea at first that the guy with glasses was Maxie. That was an interesting choice.