The first thing I thought of, is the marriage could devolve into a father/daughter type of relationship, rather than equal partners, which is what a marriage should be.

Who would your friends be? Teenagers or young people her age? Would you be comfortable as the "old man" socializing with a bunch of young people. You will be the age of the young people's fathers, or at least in their age bracket.

Is she willing to hang out with people your age? What will your friends think? Will the two of you just be oddballs in any social situation?

At least for the next ten years, you could be the target of a lot of judgment from others. Ten years down the road it won't matter so much. But at this age, there could be a huge maturity gap. And a lot of raised eyebrows from others.

She may even be unconsciously looking for a "Daddy" figure. Would you be OK filling that role for her. It would definitely bring out your caring/nurturing aspects, but it could begin to feel like you are having sex with your daughter.

I'm just throwing potentials/possibilities out there that may need to be considered. Doesn't mean any of them will come to fruition should you end up marrying or having a loving relationship with her. But, you will have an extra layer of (age related) issues to deal with, as well as the standard issues couples often deal with.