Basically all of the potential of the Facebook API can be represented by one idea: building applications on top of the social graph. Now that value proposition is dead, so the API is now little more than Microsoft Passport. I'm not really sure why Facebook is even keeping it around.
edit: To be clear, preventing private friend data from being shared is a good idea. But they did more than that, an app you install can't even see the list of your friends that haven't also installed your app. This effectively kills any app concept that expects you to have a 'friend list' screen populated even if your friends do not have the app yet. In other words, most interesting social app ideas.
apps can still get the list of friends of a user, so building on top of the social graph is still possible. (e.g. to discover friends using the same app)
I have an app that lets users mail postcards to their friends. Open app, see friend list, pick friend, enter address, mail card. This idea no longer works (and has to rely upon phone contacts.)
There are a million other ideas like this that aren't spammy/unethical/evil. This was basically the main pitch Zuck et al made for the API when it was launched, that you could bring in your friends and make your apps social. It's hard to imagine a way to bootstrap an app up from nothing if you have no way of knowing who the user might want to socialize with in the first place.
Yup. I have this principle of never investing in or building on a closed ecosystem. This means I miss on a lot, but ultimately all of these services go down with no regard to your business.
edit: To be clear, preventing private friend data from being shared is a good idea. But they did more than that, an app you install can't even see the list of your friends that haven't also installed your app. This effectively kills any app concept that expects you to have a 'friend list' screen populated even if your friends do not have the app yet. In other words, most interesting social app ideas.