This sounds right to me. I've tried lots of non-successful games in the iOS app store, and haven't found one that's much fun to play yet. On the other hand you see game developers who have sublime gameplay, have done no marketing, and succeeded -- most recently Tiny Wings.

It sure seems like fun gameplay is the limiting factor. It also seems like that should be a less scary problem to solve, but I'm not a game developer.

Is it considered hard to assess whether your games are fun? Or just hard to make fun games?