Encourage users to give feedback on each app
At present, very few users give feedback and reviews on apps. As a result, other users are less likely to try them.
At every possible stage, there should be a mechanism for users to rate the app - provide a number of stars and comments. That could be an email reminder or a message that comes up next time they visit the store or something inside of Office 365. The more users that give feedback, the more that other people will feel encouraged to try.
Be careful what you wish for...people generally don't give feedback unless they're disappointed.