Allow UI data binding to bind to internal properties, not just public properties
You should be able to bind UI Elements to properties with an internal access modifier, assuming they're in the same assembly of course. Currently you're forces to declare you're properties as public if you want to use UI data binding. This forces you to expose more of your API than may be required.
In my 747 years of developing, i have come across a place where this feature would have been helpful:
With EF, if you want to add a reference type property to an entity (through the partial class), that property needs to be internal. Otherwise the silverlight WCF data services client library attempts to serialise it and save it to the database.
Although this is really a problem with the WCF DS library, being able to bind to internal properties would have saved a lot of headaches. Rather than being able to bind to internal properties straight off though, I would rather the InternalsVisibleTo attribute be respected.
Now that's a strange request. In my 743 years of developing, I never needed such a feature.
The front end developer is the last one in the chain. He already is the one who should see the least stuff. Internal stuff is needed especially for the logic and data access layers. Internal stuff shouldn't be exposed in the GUI anyways.
If you want to expose an API to 3rd parties, use another layer (or a WebService to be concrete for most cases) to wrap the functionality you want to expose.