I don't think braces are something guys particularly look for, but it doesn't turn them away. As for what qualities they look for, it really varies between individuals. Personally, I think some important qualities are good communication, standing up for each other, and sticking together through the good and the bad. A good sense of humour is probably the most common answer, but I think that it is important because usually if you don't find someone's humour funny, you find it annoying - and why would anyone want to be with someone who they find annoying? Actually, psychologically speaking, women are attracted to men with a good sense of humour because they associate a good sense of humour with honesty - another very important quality.
Just don't worry about it too much. I know this sounds SO cliché, but just be yourself! Don't change into someone else just to please one person. Good luck :)