Absolutely nothing, except maybe panicking some passengers. Planes are pressurized, and the exits all open inward. It's not possible for a human being to overcome that force.
The odds of someone EVER actually opening the door mid flight are ridiculous. Plus how many people would stop that from happening once they knew was occurring - I know I'd be one of the first people up and trying to get on top of that person to prevent that from happening - and I know I'm not alone on that thought.
http://www.askthepilot.com/questionanswers/exits/