I think Atari may have died out after an argument with Nintendo over some copyright or something. Game Informer did a report on the demise of Atari a few months ago. Sega was going pretty strong for a while too.

So Sega and Atari would probably still be alive. That could also have kept some games away from Xbox and around half of the PS3 games. Most of PS3 are spinoffs of games from Atari, Sega, and Nintendo. That could have led to the death of Playstation. As long as Xbox got Halo and Call of Duty it would probably stay around.

I also think the evolution of gaming might have slowed down. I don't think Atari or Sega would have pushed motion gaming as much as Nintendo. Without the competition, the Xbox Kinect would probably have waited a few more years. The good arcade games might have stayed around a bit longer too.

Maybe if Sega had survived, Sonic wouldn't be a shameful shadow of it's origins. Sega would have stayed more true to the real Sonic than Nintendo did.

If Nintendo would have not existed, Sega would have most likely taken over the video gaming industry. Mario wouldn't have existed as well as many other popular video game characters.

Probably wouldn't be any video games or they would be of way less quality then they are today.