A few mechanics changed, so there were some changes in the meta related to that. There were also some new items that were major buffs to certain Pokemon, and of course the usual shifts in dominance of typings.
Hazard play shifted significantly. Hazards are still powerful, and there are new hazards (Sticky Web, a ground-contact hazard that lowers Speed when a Pokemon comes into play!), but the move Defog was changed to clear all hazards in play. Defog is impossible to spinblock and can only be stopped with Taunt, which means that it is now harder to ensure that hazards stay down.
Certain types shifted in effectiveness. The weakening of hazards and the introduction of some premium-grade STAB abusers has made Flying attacks very brutal and common; Mega Pinsir and Talonflame are the primary threats here, and they will rip through anything with a Flying weakness. Steel's tanking potential took a hit, as it lost Ghost and Dark resists; this also means that Steel/Psychic Pokemon like Jirachi and Metagross are more vulnerable to those types of attacks. Dark and Ghost attacks are thus more common; Ghost is another threatening STAB due to this. Dark type attacks will also be fairly common, because a buff to Knock Off's base power ensured that virtually anything capable of carrying it will be. (It now hits for 95 BP)
Weather got a huge nerf and is no longer infinite. Now, weather-setting abilities only initiate weather for 5-8 rounds, depending on whether the user is holding the appropriate weather-extending item. This means that weather-centric teams are now more uncommon; you can still expect to see them however, especially Sun team, which use the devastating Mega Charizard Y as a combination setter/sweeper that makes Gen 5 Ninetales look like a kid with a bottle of hairspray and a lighter.
New items to watch for include the Weakness Policy, which gives the user 2 stages of Attack and Special Attack boost if they take a SE hit of any sort, and the Assault Vest, which disables any non-offensive move (basically a permanent self-induced Taunt) in exchange for a one-stage Special Defense boost. You'll often see Dragonite abusing Weakness Policy with Multiscale to set up a sweep, and Conkeldurr is a common Vest user.
Not much else to say besides the introduction of Mega Evolutions. Most of the drastic meta changes, such as rain teams becoming so much less common, are related to mechanics changes.