I've given this a lot of thought, because I'm working on a couple of vaguely similar projects. My conclusion so far is that pretty-printing is great, but enforcing structural constraints in the editor (beyond perhaps something like ParEdit) tends to cause more trouble than it's worth.
Consider the transformations you might do on a piece of source code while you're playing around. A lot of these (for me, anyway) are things which change not just position or name, but the part-of-speech of words, or temporarily make a nonsensical expression. (I do it in my shell a lot, too.) Rigid structure-enforcing editors hinder this.
So for Lisp, either you end up with something that's less useful than Emacs, or something that's just a thin display layer on s-exps (which is fine, but not game-changing) and then users have to understand s-exp editing anyway. The fact that the names of special forms, functions, and variables are all symbols in Lisp is not mere coincidence.
This highlights one of the big frustrations I have with most programming languages: they optimize for making a section of code look good and concise, but as a programmer I want a language that also optimizes for my changes being concise. For example, C# can look pretty decent in many cases, but changes which are conceptually simple can be huge diffs. Quoting an expression in Lisp is literally one character, but it takes 5 lines to quote a print statement in C# " rel="nofollow">http://msdn.microsoft.com/en-us/library/y2k85ax6.aspx>, and anything nontrivial grows painfully fast. This is why it's impossible for me to truly assess something by looking at sample code, or a screenshot -- so don't take this as a criticism of Pretty Lisp!
It could be that as we move higher up the abstraction scale this becomes less important. After all, Mel wouldn't be happy at such a high level as "characters". But I think there's something fundamental about the concept of quoting symbols -- 'using something as something else' -- whether those symbols are bits or characters or round-rects. The #' in the screenshot here looks out of place, as if the author didn't know quite what to do with it.
I look forward to see what the author can make of this (new ideas and experimentation are great!) but I don't see anything here yet that jumps out at me as revolutionary, or must-have functionality.