A lot of the biggest Web successes in the past few years have been services that have achieved some sort of "check-in effect." By that I mean their users are actively checking in to their services for updates. Check the latest deal. Check my activity stream. Check on my farm. Check if there are more updates or comments.
This seems even more the case for mobile successes, where more traditional traction verticals (e.g. email, adwords) are less useful. The successful apps (more in the non-game space) make the home screens often because people want to check them repeatedly and often.
A lot of services could be recast slightly with the check-in effect in mind. For example, taking data sources and chopping them up into streams and push updates in some useful fashion, or highlighting trending topics. I've seen this most noticeably in LinkedIn's redone app.