In no particular order
There are lots of nice glossy icons but the initial impression is spoiled by the old style "P" graphic. Other than that it looks nice and the js is smooth. If you're going to use a single page for all the content you lose the ability for people to bookmark anything other than the index.
If you're going to do a js nav content swap then why not swap the whole page. you seem stuck between wanting to keep the initial content fixed on the screen and using nav to show more. Reality is, you can see the initial content once you scroll down so why not just swap it out, like all the other content.
You have navigation tabs at the bottom of the screen. As the page doesn't reload a user may not see anything happen if they on a link and the content is changing in a lower div. That happened to me and took me a few moments to work it out, and only did because I saw a tiny bit of the div that was changing.
Clicking the demo takes you into a version with no link back to the main page (and your buy button)
I can't read anything in the gallery images, some 100 crops of key features might be nicer. Blurry screen shots dont offer much.
Everyone knows what the F and t buttons are. You don't need Social Media Links in MASSIVE text under them.
As it's quite an involved process I'd suggest making a desktop recording of you creating a proposal.
I clicked the link to tangle in ie 8 on win7 and got a jS error (on tangle.ca), "cufon is undefined". As you're selling to web devs I guess you have less margin for error.