For a team of 28 people (see team photo on http://www.rockmelt.com/thanks.html) I was kinda expecting more than what I downloaded, especially as this is just a skin on top of Chromium.
All of the heavy lifting in terms of the browser itself has been done for them by Google.
From a cursory look, it appears they wrote all the new functionality in C++ (and ObjC for UI on the Mac). That's a lot of work. Why they didn't do the bulk of it in JS/HTML/CSS is anybody's guess.
All of the heavy lifting in terms of the browser itself has been done for them by Google.