I just picked up RWC 2011 today, simply because it was only a few dollars brand-new, and have only played a couple of games. It is definitely a very minor iteration on the Rugby '08 formula. Comparing the 2 games, I think RC captured the free-flowing, dynamic running game well (yeah offloads and crazy passes aside), and has practically no forward play; RWC on the other-hand has pretty solid forward and ruck play mechanics, like the pick-and-drives, a dedicated button for a forward hit-up from the ruck, quick steal buttons etc, but pretty 'rigid'-feeling back-play and passing mechanics.
Definitely a combination of the 2 games could be a great step forward, or at least whoever creates the next instalment of their franchise should take a lot of leaves from the oppositions book in terms of gameplay mechanics.