There's a common but often-ignored meta-lesson here - begin by describing what the thing actually does, concretely.

"An exploration in messaging innovation being led by the team responsible for Thunderbird, to explore new ways to use Open Web technologies to create useful, compelling messaging experiences" has near-zero information content and happily eats up the invaluable initial attention and visual spaces. One has to scale up a substantial wall of text before reaching anything specific but not before being invited on a detour into the 'Guiding Principles'. I didn't go down that path but presumably the principles include 'Be vague'.

It may sound like mere grouchy nit-picking but I've found that this type of failure at basic communication in a project's description is often indicative of its future lack of success. The pages of the Chandler project used to be a wonderfully instructive example but even they seem to have cleaned up their act on their current website. A very incomplete list of words and expressions to avoid before clearly stating what a product does -

revolutionary

innovation

vision

open

passionate

watch a video/screencast

Any others?