Nice stuff here. Working up a new collaborative blog with 2.0 which allows parallel of finding out about new features and the issues of collaboration in such a space.
One thing that seems a little strange in the Roles is that you can allow users to publish, but then only Editors or Authors can edit_published_posts, which coincides with the ability to edit_others_posts. It seems that at Author level, perhaps, it would be sensible to allow said Author, by default, to edit their own posts, published or otherwise.
I appreciate, from hints offered here and there, that plugins are likely, and that roles might be customisable, but for many (arguably), some sensible "out of the box" defaults should be in place. This one *seems* a little out to me, but I could be missing something!