My initial reaction: cool idea, I've seen it done a couple times before, what does this offer that's different? I wasn't able to get that from the landing page so I watched the video.
Watching the video, I was excited because if I could make a good looking proposal without much effort, that'd be helpful. But from the video I got the idea that there isn't much functionality and the output isn't on par with my own quality expectations.
Here's what I would've like to see:
1 - A few real live demos. I don't like watching videos, and I don't see examples in the live site. I just want to answer my first burning question of "How cool is the output?"
2- What other tools do we get in the editor? Some basic things like a table of contents would be mandatory. Something like a slider would be nice, but then you're becoming a bit of a Wix clone if adding a bunch of html widgets.
3 - Why not make it free? You need feedback more than you need money at this point, IMO.
4 - The branding, the name, is solid.
Your killer feature will probably come from the feedback you get. If I had to guess, it'd be an integration and not something 'sexy.' Agencies I work with already have systems they follow and a product owner wouldn't swap to a service unless it either provided massive value, or was able to read PDFs or Google Doc files and make them look good on the web.