New info! The new Mewtwo has been revealed to be a '
Mega Evolution', and it's not the only Pokemon with one, though not all Pokemon will have one. It would appear they need to hold a 
Mega Stone to achieve the transformation, and the forms have different abilities and sometimes stat alterations, or different typings.
Mega Stones are a type of item like Evolutionary Stones and include specific items such as 
Blazikenite or 
Lucarionite. It would seem that the Blazikenite is not found within normal gameplay. Mega Evolutions are temporary, being only in battle.
New video here!: 

Mega Blaziken (has Speed Boost), 
Mega Lucario (has Adaptability and increased attack), 
Mega Absol (has Magic Bounce and increased attack & speed).
Mega Mawile (has Huge Power and increased defense & special defense). It and normal Mawile are now 
Steel/Fairy type.
Mega Ampharos (has Mold Breaker) is shown here, and it is an 
Electric/Dragon type. 
Mega Mewtwo has the ability Insomnia.

Another new Gym Leader, 
Korrina. She apparently holds the key to understanding Mega Evolutions. She is also the Gym Leader of 
Shalour City, where the 
Tower of Mastery is.

And some more completely new Pokemon.
- Gogoat's pre-evolution, 
Skiddo. It is a 
Grass type, and has the ability Sap Sipper.
- 
Bunnelby, a 
Normal type. Has the ability Pick Up or the new ability 
Cheek Pouch. If a Pokémon with this Ability eats a Berry during battle, it will not only reap the benefit from the Berry, but the Pokémon will also be able to regain HP. 
- 
Dedenne, an 
Electric/Fairy type. Has the abilities Pick Up or Cheek Pouch.
Also it seems they're finally acknowledging EVs/Effort Values openly now with 
Super Training: 
serebii.net/xy/supertraining.shtml