An issue that I've come across when trying to use WebLaF is that it defines custom versions of pretty much every Swing component, this makes it difficult if not impossible to use with any other components from different libraries.
For example if I want to use JXTable from SwingX or SortableTable from JIDE with WebLaF then I'm basically bang out of luck: it's one or the other.
As a Look & Feel WebLaF should not mandate the use of custom component classes for core components (obviously the various extended components are a different matter). Most of the subclasses only contain helper methods (often just 1-line redirects to other helper methods!) which could easily be moved to static methods on a helper class instead so the changes, while breaking for current WebLaF users, would be fairly trivial to make for the most part.