What I find to be X/Y's most fascinating feature right now is how we're actually updating old Pokemon with new stats. Besides the Special Split in Gold/Silver, we've never seen GameFreak alter a Pokemon's stats for a new generation.
Serebii is currently checking the older Pokemon, and has come up with a preliminary list.
Also... I need two more Dittos (well technically 3, but I really don't need a neutral nature). -_-