I've noticed that visitors can preview Open apps if they're not signed in to Yahoo. That's great! It removes a barrier to entry.
However, when someone's logged in to their Yahoo! account, they only see a screenshot and don't get to actually use the app. Why penalize Yahoo! users? Why not let them poke around in apps too before authorizing them?
For an example of an Open app that allows previewing, sign out of Yahoo and then visit:
http://apps.yahoo.com/-DnFMcF76/YahooFullView/