EndNote is far behind Mendeley in having an inuitive, modern UI. Recent upgrades modernized icons but not the actual user interface. One of the big elements that makes me keep missing Mendeley is how opening papers from the library is handled. In EndNote, each paper is a new window and they all collapse to the bottom when minimized with tiny titles that make it hard to know which is which. Better would be to make the library a tab, then each open a paper a tab. Also, in the current window for viewing a paper, the reference is one tab and the paper PDF another tab. This makes updating the citation information difficult when working from the PDF to get the information. Rather, when looking at a paper the PDF should be open with a pane on the right where the reference information can be edited.
Basically, shamelessly copy Mendeley for the love of God. Else, I don’t see EndNote remaining a viable product.