Its interface is what I find most impressive about it, but functionality-wise the commenting and reviewing features are really nice -- there's a floating icon t

Its interface is what I find most impressive about it, but functionality-wise the commenting and reviewing features are really nice -- there's a floating icon that lets you nestle comments into the margin and if you click on a comment it spotlights the region of the text it refers to. I've been in a lot of situations where this sort of thing would've come in handy, and combined with the other collaboration features they're introducing, it could make for a pretty great setup.