From what I have seen copy 80% and innovate on 20% seems like a popular rule (ps: 20% seems very low but it is pretty noticable). I quess it makes sense, if I would start with a new game I dont want it to take hours to learn the game's manual and am drawn when I see an gameplay element in a game that I really liked before in an other game but also dont want to play the same thing over and over again. Ofcourse this differs probably alot between people. E.g. some find X3 (the space game) one of the best in the space sim genre, and others find it to be to much of a hassle to learn. I think it mainly comes to down to whether you want to focus on a niche market or not.