Drag and Drop UI consistency
The Drag and Drop UI seems clumsy for files into libraries. It seems you have to position the dropped item right on the dividing bar between the library header and the document list for drag and drop to work (at least in IE 11, in chrome dropping anywhere on the library seems to work). NOTE that the prompt at the top of the page says drag to any location but it doesn't really mean what is says, since the drop only works for the library itself. Not for example to other libraries listed on the nav bar.
I'll add my comment here since my issue is a subset of inconsistent UX for Modern libraries when it comes to drag/drop tagging. Drag/drop tagging works great for dropping into the primary group in a library and the UX makes sense (there's a gray box around the group you're dropping into and little tags appear in the far left column), but dropping into a secondary group has no UX at all - there is no gray box, and there are no tags, in fact users would have no way of knowing that they are not just uploading a file to the library but also tagging it twice (once on the primary group and again on the secondary). Oddly, drag/drop tagging does actually work in Modern, but from a UX there's no indication that this feature is even available.