As Weekend Gamer points out, the "easy to play, difficult to master" description so often used by professional reviewers doesn't mean the same thing to non-gamers as it does to veteran gamers like us.
Veteran gamers typically have no idea just how much our gaming experience has trained us for present games. I know people who were gaming addicts for many years, and a mere 3-5 years away from gaming crippled their familiarity with gaming controls and goals. They can skillfully play an RTS and a sports game because the control scheme's closely match games they've played before. But something unfamiliar, like controlling character movement and camera movement independently in Halo 3, is beyond frustrating.
People who didn't play video games as children have an even harder time. A good eye-opener is when a game says "Press Start" and you have to show the non-gamer where the Start button is (that small, camouflaged button in the middle, with "Start" written in camouflaged print so tiny that most 40+ folks probably have trouble reading it without glasses and optimal light).
Anyway, there are obviously many implications to this, most of which have been discussed many times before. But one I'd like to point out is that I bet this is a major reason for the popularity of Flash gaming and similarly simple internet games. People don't get stupid with age. Old folks can learn complex new systems, like boardgames. But learning control schemes and unfamiliar concepts (video game goals and strategies are not perceived like boardgame goals and strategies) is different and difficult.
Flash games are the door through which a lot of non-gamers are currently passing to enter the world of gaming. I don't think they're lesser games. I'm just saying I think a lot of non-gamers perceive them as an undaunting way to familiarize themselves with this still-new passtime called video games. A person's first game has to be dirt simple, because the mere concept of interactive entertainment through a monitor or TV screen and controller is foreign to them. There's more to learning Pac-Man than "up, down, left, right", eating the dots, and avoiding the ghosts. The medium itself must be learned.