But building a good app on all three major operating systems is not solved by an abstraction layer, neither by a cross platform gui toolkit nor by a web layer.

But building a good app on all three major operating systems is not solved by an abstraction layer, neither by a cross platform gui toolkit nor by a web layer. Different operating systems have different conventions, metaphors and standards. A portable application will usually feel foreign on all, but the developer's primary platform. Unless, the developers invest in adapting to the differences of the platforms.