According to this GameSpot article Blizzard will be limiting Heroes in competitive play to one per team.  

Personally, I think this is awesome, though I know many people will be concerned, outraged and perhaps even running around like schoolgirls with bats in their hair. 

There are a few situations where I can see it being problematic (Your Pharah is horrendous and you just need to swap out temporarily to kill the Bastion for example), but overall I think this is a brilliant move. 

Not only will it remove the venom that inevitably spews forth when the second or third Hanzo is picked on your team, but it will create more rounded lineups and ease the pain of any Hero that becomes overpowered throughout the lifecyle of the game. 

I've been wrong before, but hopefully I'm right and this is a fantastic change for the game.