Complex answer: It really depends.. If the person was someone I could honestly call my "soul mate" - then I would still be willing to give it a shot. I mean, I've watched my sister go from massive amounts of weight (in middle school) to half that weight (and a LOT healthier, BMI 28%).. all for herself, thanks to the support of a man who loves her.
People do remarkable things if given the chance. But I'm not about to go and break my standards for just anyone.. You have to show me your worthy of that "bend" in my preferences..
I don't believe that looks are the best qualities that a person can have. But If I hit it off with someone and then we met in person and she didn't have the looks that I was thinking she should have I would still be attracted to her inter beauty.. and that should not make a differnecein the person that you really liked while talking.. its all about the person not the looks all the time. So to me its can't be a deal breaker..
Looks can always be a deal breaker, but it is more about the complexities of chemistry than just the sum of individual body parts. I know I am open to a wide variety of looks in men, from their ethnicities to height and body shape. Even with that, I have talked to men online that I had some basic idea of their looks based on pics they posted, but when meeting in person I just did not have an attraction to them. It wasn't just looks, but overall chemistry that killed the chance for date 2.
So, talking is great but in online dating it is important to meet sooner than later to see if there is really a connection.