Nintendo has announced plenty of new features for Pokemon X and Y in the last few months, but hasn't shown them off. A new playable demo at Gamescom took care of that, serving as the public debut for some its recently announced changes.
The 10-minute video (via The Escapist) shows how Mega Evolutions work, with Professor Sycamore letting you try it out on a Mewtwo. It also briefly shows off riding a Skiddo Pokemon, which may replace the bike since the functions seem similar. Finally, the demo showed that you can decline a battle with other trainers, rather than automatically being entered into one as soon as you're spotted.