IMO open source projects need a critical mass of developers to achieve "polish". All the edge cases and so on that isnt' a matter of writing clever code, but sitting there for days writing tests or manually investigating corner cases, or putting in resources for good crash reporting, seamless updates and things like that. I have yet to see an open source project that deals well with this (Firefox doesn't count because they have a lot of $$ and can employ devs to work on all these things.)

When you reach a scale of millions of users, all these tiny details start to affect a sizeable amount of users.