There's a great roundup of iPhone 5 Rumors on this very site. That should cover most things. As for what's actually going to be new, only Apple knows for sure until they say so on (or around) September 12th.
Physically from all the leaks (granted not confirmed so no real way of knowing) - iPhone 4/4s thats .3 of an inch taller and slightly thinner. I really hope they widen the screen or else I'll probably be jumping ship after 5 years with only an iPhone