Nothing. I eat my lunch at 11:00 - couple of slices of cheap pizza - then a double-helping of dinner in the evening. Eating breakfast just makes me feel sluggish. On the weekends I do it.

For my 30-50 minute (depending on route) commute, I don't notice a difference from having breakfast inside me. For a long recreational ride - anything over a couple of hours - I definitely have to eat beforehand.