I haven't played games regularly since SNES, but my kids are all over PS3 games.
My experience as a fuddy-duddy Dad who picks up a controller every now and then is I can't even get past the training levels...so there is a base level of
skill that needs to be there that I don't have (or have the patience to acquire). This is true on Assassins Creed, MW2, and a few others that go bang and splat.
I think the gameplay has to be easy enough that a 1st timer with a reasonable amount of patience could pick it up. If you have been playing for 10 years,
that will probably make it seem facile. I know my kids can get through a game in a weekend if I let them....but I'm sure I couldn't get through those same games
in 2 months (assuming I stay employed, married and otherwise continue as a productive member of society)