If you find yourself continuously riding in the 39x12 combination, you can save some wear and tear on the drivetrain by shifting to to the big ring and adjusting the rear appropriately.
Compact cranksets can give you more flexibility: you can get chainrings to fit a compact crank that are "normal" (nominally 39x53) so you can match gears with everyone else, but if you ride in lots of hills they let you put a smaller front on than standard cranks (most have a lower limit of 39, maybe 38). A standard compact crank setup is 36x50 front, 11x21 or 11x23 rear. You get a little lower gearing for the hills, and don't lose all that much on the upper end.
For my old legs and riding style, I wound up with a 38x52 front, 12x25 10spd rear. I'm not racing so being able to get up the hills a little easier is nice.