I think it's a combination of factors. But low barrier to entry means anyone with $99 + a Mac can try. That combined with the low cost of games also prevents

I think it's a combination of factors. But low barrier to entry means anyone with $99 + a Mac can try. That combined with the low cost of games also prevents big budget titles from dominating. While there is still room for a relatively higher budget games (sold at $9.99), companies simply can't poor millions and years into titles and expect to make their money back. Also as much crap as the App Store gets, it's very democratic in that everyone is equally screwed by the volume of apps. But it means that unknown IP seems to have a chance to rise to the top as well.