From an interview with lex Rigopulos, co-founder of Harmonix, the company that developed the game:
I do think that during the coming years we will continue to try to bridge the gap between simulated musicianship and real musicianship. That said, the path there is not obvious: As the interactivity moves closer to real instrumental performance, the complexity/difficulty explodes rapidly. The challenge is to move along this axis in sufficiently tiny increments, so that the experience remains accessible and compelling for many millions of people. It’s a hard, hard problem. But that’s part of what makes it fun to work on.