Geomajas

I really like the enterprise level capabilities for server side integration with different business domains, full fledged security and proper configuration (Spring).
the integrated client server architecture is extensible through a plug-in mechanism and the OS project has an inviting set-up to write your own.
GWT is a big plus compared to dojo!
Documentation is decent but can be improved further.
Go Majas Go!

Finally a client-server based web GIS platform that makes use of the Google Web Toolkit, to get rid of all the cumbersome Javascript plumbing. It's Java all the way!