Drag and drop is popular. We are just less and less used to the paradigm, and don't expect it to work in the browser. I don't think there is much wrong with the approach, but there is a blur between app/OS interfaces and web ones. For example if that widget kit was a separate window like say the old Gimp toolset, it would feel okay to me.

The placement on the RHS is a little awkward, especially if there is scrolling required.

The commit process was confusing for me in trying to publish a page, but I found the CMS more intuitive than something like Joomla, that I feel out the box is a train wreck (it takes a good while to fit the pieces together/learn how to use).

There's scope for better CMSs that's for sure.