The svn_delta_editor_t interface has served Subversion well from its earliest days, offering a unified mechanism for describing how to transform a tree of files and directories and properties into another tree of the same. We use it commit changes from client to server; we use it to update/switch/merge/diff/etc. changes from server to client. But it comes with a set of driver constraints that we now feel aren't able to meet the needs of a more mature Subversion. So, in short, this critical piece of Subversion's design in need of an overhaul.