The ten minute demo shows how battles, riding Pokemon, and mega-evolutions will work.
Nintendo have made a 10 minute English demo of Pokemon X and Y available to play at this years GamesCom. While it doesn't show us much that we don't already know, it does do a great job of showcasing the game's new features, such as riding Pokemon and mega-evolutions.
The demo starts off with the player having to go find the game's new Pokemon professor, Professor Sycamore. On his way there, the player encounters some wild Pokemon and some Pokemon trainers. What's interesting to note is that unlike previous installments of the game, where you must battle a trainer if you "lock eyes," in Pokemon X and Y you can actually decline the battle if you wish.
When you meet the professor, he'll explain about mega-evolutions, and give you a Mewtwo with a mega stone so you can try it out. From the looks of things, the mega stone will appear in the combat menu just bellow your Pokemon's standard 4 attacks, and you will still be able to attack after mega evolving.
At the start of the demo, the players' Pokemon are all level 30, and don't seem to gain any experience, while the final battle with Professor Sycamore sets all Pokemon levels to 100. As such, it would seem that this doesn't appear to be a "normal" part of the game, most likely a scenario that was built purely for the demo's sake.
The demo also briefly shows how players will be able to ride Pokemon like Skiddo. It looks like ride-able Pokemon will replace the bike in this game, as their mechanics look pretty similar.