Traditionally web/ria frameworks/toolkits have been fairly monotilic: even though many of the frameworks are designed to be extensible, it has been painful to build, package and distribute a new user interface component. This have lead to situation where framework vendors mostly bundle all the functionality inside the framework - in order to make it easy to be use.
As as result - those brave developers who have built and released add-ons to frameworks have been "second class citizens" from the point of view of framework users - it is simply just so much easier to use built-in functionality than test and integrate third party add-ons. Partly because of this - many of the add-on ecosystems have not been too lively and users mainly had to live with whatever functionality the framework vendor has built into the framework.
I hope that the opening of the new Vaadin Directory will change the situation - at least for Vaadin Framework. In order to show how easy and game-changing it is to use add-ons with Vaadin, I recorded a short (4min) video:
(it is recomended to watch is in full-screen and HD)There is also an old screencast on how to create a new add-on package that shows that building and packaging is almost as easy.