Multithreading would be a great feature, working with an editor where the whole UI tends to block itself on some network (tramp or via sshfs/nfs) operations is unpleasant. It's also a sign of its age. I also miss, for example, UI localization. You can, of course, have everything hardcoded in English, but it's a limitation. Well, whole UI is dated. In (g)vim you can have nice pop-up code-completion, in emacs, it's not that nice. The UI is very dated, sadly.