I'd actually like to see the exact opposite: an editor that warns me every time I use a too common and worn out phrase or sequence of words. That shouldn't be too hard to do, should it? :)
The other thing it needs (which I also wish I could use more precisely in Google search) is some concept of punctuation beyond the occasional comma; playing with it seems to result endless run-on sentences. But hey, early days.
I wonder if/how it selects for linguistic quality? While it won't be too useful to the general public if it defaults to the dense academic language of textbooks, I too was a bit startled by the frequency with which 'I'ma let you finish but Beyonce had the best syntactic parser of all time' keeps popping up.