Though it's too early in X/Y's lifespan to say how the metagame will stabilize in the future, it looks like Game Freak made a very good conscious effort to balance out the extremely offensive and heavily centralized playstyles of Generation V.
- Weather, which was easily the most defining aspect of B/W, was heavily nerfed; this alone could control the incredibly powerful weather offense that made stall teams incredibly difficult to maintain.
- Though there were more powerful threats added in the form of Mega Evolutions, only having one of them per team and not allowing them to have items somewhat balances them out. (And it seems like many of these were designed to be legendary/Uber status anyway, so being overpowered may have been the entire point.)
- Fairy-types were added to neuter the threatening Dragon- and Fighting-types running amok. Both adding new Pokemon with this typing and readjusting the typings of existing Pokemon has made quite a number of Pokemon viable.
- The small number of new Pokemon, abilities, and moves seems to have been geared more specifically toward countering or checking a lot of the overpowered threats from Gen V, both offensively and defensively, without completely flipping the existing metagame upside-down.
- Changes in the game mechanics, such as sleep reverting to pre-Gen IV mechanics and having some types immune to certain statuses, also bring some balance to weaker types. For example, the defensively poor Grass-typing has become slightly better in being immune to a lot of common status moves.
- It seems like many of the new Pokemon that have been added are neither too overpowered nor too underwhelming; there are good discussions going on!
- Small but perhaps significant downward adjustments in many existing moves such as reducing the base powers of the elemental attacks (Fire Blast, Ice Beam, Hydro Pump, Thunderbolt, etc.) and allowing for some offensive holes in Substitute, arguably one of the best moves in the game, have buffed defensive styles enough without completely dooming offensive styles.
Hats off to Game Freak for at least attempting to make Generation VI both fun for the casual player and balanced for the competitive gamer. Who knows where it'll end up, but it looks like it'll be a lot more diverse than what Generation V turned out to be.