That looks awesome. For most of the video I couldn't help but think about how I could do the same thing on Google Docs, but your "Export" and "Simulate" features are obviously your money makers. And I would definitely pay for this product and use it a ton. Great work!
From a technical perspective, great choice sticking to browser technologies instead of native apps. I think it fits your product extremely well and I love seeing HTML5 technologies being pushed.
One piece of advice - remove the bit about not having your computer so you have to resort to paper and then transfer to software when traveling part. There is the same issue with your product as you've described it. So unless you're planning to add some easy transfer option, remove that bit.