Subversion's missing piece is the patch-review system, agreed. Rietveld was 11.5 years ago (and Mondrian was 2 years before that - Perforce admittedly), so we have plenty of things to be inspired by. In the last year a "shelve" facility has been perfected for Subversion - https://cwiki.apache.org/confluence/display/SVN/Shelving-v3+in+Svn-1.12. It seems to me that this could be grown into a patch review system.
Ref: https://en.wikipedia.org/wiki/Rietveld_(software)
