I'd like to voice additional support for two-column export. Almost all journals to which I submit manuscripts have their final typesetting in two-column mode. For me, editing in some kind of multi-column mode isn't necessary as I edit in single-column mode using my current editor of choice, TeXShop. What would also be useful for me, once two-column exports are in place, is the option to have figure span the columns.
In general, to make Manuscripts truly useful to me is the ease in transitioning from the writing/editing stage to submission, say to Physical Review journals. Currently, with TeXShop, I work directly with the LaTeX file that's finally submitted. I believe that the Manuscripts editing environment is, or will shortly be, superior to TexShop, but if too much effort is required getting the exported LaTeX file ready for actual submission I can easily imagine I'll just revert back to my old workflow.