Too bad you can only vote once, i would put a +20 on this !

That's for me the critical feature #1 in a service : your users use your product for one very clear reason. They come to your app to do specifically this. There might be other features on your app, but there's a main one, clearly defined.

And by using this feature, they get value. It is simple, it is straightforward. Value can be content (Quora answers). Value can be an action or reaction (check-in, "count me in" on a plancast). Value can mean earning or saving dollars.

Love the subject, blogged about it here: http://laurentk.posterous.com/in-search-of-the-perfect-viral...