What’s wrong – how can I find out what Eclipse doesn’t like about my project?

As you can see it shows no errors in sources, no other errors, just the project. There is no tooltip telling me what’s wrong. Can’t see anything wrong on console either. Project builds fine with build.xml.
What the hell Eclipse wants from me and my project?
Go to Window -> Show View -> Problems (alternatively Alt Shift Q, X)
This window should show a description of the error.
It’s likely some problem in the build path or one of the dependencies of your project. If a dependency cannot compile, your project will not compile too, although there is no explicit error in its code.