The major difference, at least with Shimano and SRAM, is that the high end group sets get new features/improvements first, which then trickle down over a few years into the lower levels. 7900 came out a while before 6700, and that came out a while before 5700. 9000 is the New Hotness for Dura-Ace, but there is not yet a matching group at the Ultegra or 105 levels with the new features. SRAM doesn't quite follow the same pattern, but the effect is more or less the same. ZeroLoss on both shifters used to be Red only, but now it's also in Force. This years Force is essentially last generation Red. 2012 Red got a new hood/lever shape and the Yaw FD which wasn't available in other groups until the just released Force22.