i've been thinking about this ever since i purchased my copy of pokemon Y. When next year's pokemon world championships happen, what effect do you think mega evolution will have on the rules? Do you think they'll allow Mega Evolution in the competition? And also do you think some new changes to the game in x and y will also effect competitive rules?

i don't see them as much of a problem since they need to hold whatevernite to mega digivolve 

It wont be broken since you need to hold an item to Mega-evolve.

Plus, you can only have one Mega Evolved Pokémon in your team.

With these into consideration, they aren't broken and they may not affect you that much. Some Mega Evolutions are going to be too strong, such as Mega Blaziken. Those strong Mega Evolutions will be pushed to Ubers. 

It's not broken because the Mega stone is a hold item,which put the player in the disadvantage of not having a berry,focus sash or any other item for that matter

 

It gives the Pokemon a massive stat boost,but Mega evolution is not impossible to beat

 

Not to mention that you can only have one Mega evolution in your team,so you still have to choose carefully which Mega evolution will help your plan best

VGC-wise, no bans.

 

As for competitive playing outside of that, some will be banned. Ofc Blaziken will. (Probably both) Only Talonflame counters it, perhaps checks it but Blaziken can always predict and nuke your incoming bird. (Talon's defenses aren't even great to begin with) Unsure on others such as Mega Gengar. We'll have to see once Pokebank comes out.