Keep it simple: don't build a product that has it all. Killer apps often have only one amazing thing for which they're known, that's it.
If you need to do multiple features, like a multimedia player and a social network, make one and build upon the first success to introduce the other.
Think about it, people will remember you for one thing only so make it simple for them to talk about you. That's the philosophy behind Business Ideas, the only website where you can test your idea before you start.