Why isn't this mesh not appearing properly?

I've been exporting all my meshes into unity without problem, but now I can't export this particular mesh. I have 45 different version of the mesh files (I save after everything worthy edit), and it breaks at version 11. The only difference with version 10 (that I know of) is that I a few polygons here and there.

Also, I never used to get that ''import settings'' tab. I think that may have something to do with my problem. Perhaps there's an error somewhere along the importing process? All I've ever done is put the .blend file in the Unity folder and it has worked just fine.

Anyway, ideas?

Thanks a lot!