Mega Garchomp: It's a thing. One of the biggest terrors in the Pokémon metagame has gotten a Mega form. God help us all.
In addition to this revelation, it seems that Mewtwo will be getting a second Mega form called "Mega Mewtwo X". Presumably, this makes the previously-revealed one Mega Mewtwo Y.
Furthermore, we've had more reveals of normal monsters that will appear in Pokemon X/Y, including evolutions for the three new starters, a Normal-type called Torimian, a Psychic-type cat called Nyaonikusu and, finally, Chigorasu and Amarusu: a pair of fossil Pokémon.