Can Roger Federer keep it up? It'll be fascinating if he can, interesting to watch who makes him look mortal if he can't.
Will Rafael Nadal continue to improve on faster surfaces? This directly relates to the previous question. At 19, Nadal still has plenty of time to prove he's got something more than clay feet. He went 50-2 on the soft stuff in 2005, but showed some hardcourt chops, too, with a 28-6 record and three titles on that surface.