Racket makes it easy to fire up a Scheme and start doing work. It's the flexible, usable lisp with libraries that I was waiting for. There's less cause to drop SICP from the curriculum when you can get a high-quality scheme distribution and tooling on several platforms :)
They should improve out-of-box usability. You shouldn't need to pick a language when you first install it - that's a barrier to new users. Have a default (the shapes one described in the getting started area would be fine), and make it easy to change.
The teaching languages are only a menu option away. Most of them are designed to be used by students working through 'how to design programs' (htdp.org). I wouldn't really consider that a barrier to entry. Keep in mind most of the teachpacks implement reduced scheme implementations designed for students at a particular level so they would not be much use to anyone else as the default startup option.
Sensible defaults. Sure, it's a polish issue. Polish is a good thing, particularly for racket which aspires to and is succeeding at making scheme accessible.
When you fire up rebol, perl, ruby, python, awk for the first time, they don't send you off to poke around in preference screens. Type and go.
My assumption was that most of the people who load up DrRacket don't go poking around for the teaching languages. It's a pretty useful IDE, not just for students. I usually use it unless I'm working with a large number of files in which case I prefer emacs.
I used PLT Scheme (now Racket) 8 years ago to work through SICP 2nd Ed. Was great fun. Should be even easier now that Racket has concurrency constructs.
They should improve out-of-box usability. You shouldn't need to pick a language when you first install it - that's a barrier to new users. Have a default (the shapes one described in the getting started area would be fine), and make it easy to change.