Thanks Eli, we've actually been shipping for the past several months but it's good to get such a large new feature out the door none the less.
This is great feedback, we've definitely struggled with how to present building up a document in HTML to people who don't understand HTML. If you're purely a designer or a product manager you'll most likely to use the other aspects of Easel which allow you to position elements absolutely and style their CSS properties with the inspector on the right.
The bootstrap feature is aimed more specifically at developers and designers who are HTML and CSS savvy. We've found that there are many people out there who are interested in designing responsive application. While they can "design in the browser" with code they've found into inhibiting their creative process.
We've built this feature to allow those savvy designers to get back to what they do best, design and not code. It sounds like we're in the right direction and with the right mix of introduction, learning and clarification it will be alot easier to use.
Thanks again for taking the time to try things out and for giving us such honest feedback.