My own editor is written in Ruby, and so all extension is done by loading Ruby code into the running process, and I can drop into the Pry debugger with a keypress, or another keypress gives me a prompt to enter a single-line expression instead. The latter is literally a one-line method. Adding a binding to eval() a whole buffer would be equally trivial... Being able to extend everything trivially in a language I'm comfortable with (so not Emacs lisp) makes such a difference to usability.
Incidentally, the ability to interact with the open buffers using a script also from outside the editor is another thing I love as an extension mechanism for editors - an idea I first saw in FrexxEd (co-written by the founder of Curl) for the Amiga, which exposed the open buffers in the filesystem (think the Amiga equivalent of a FUSE filesystem), which would have the added benefit of not being language specific. It doesn't need to involve any FUSE-like stuff either - just a command line utility to "cat" an open buffer and to replace the open buffer from stdin would be sufficient.