This is going to sound a little misguided, but bear with me. If you want to see what you're missing: Take a look at the IDEs of the moment. Try something like eclipse on java code or maybe devstudio in the windows world.

In general, vi/emacs are missing support for editing above the level of text (eg. refactoring) and support for UI above the level of a text terminal. I know add-ons exist for both editors that do some of these things, and of course, you can run both in a graphical terminal with custom fonts, etc -- but if one is being honest, it's not very well integrated in either of the old standbys.

I'm a vim user, but not really an advocate. I'm just used to it, I like the way it works (mostly) out of the box[1], and I think it's text editing facilities match the way I edit better. It's a great generic text editor -- as is emacs (for some others, not me). But all they do is understand text...

I had to do some Java development recently. So I tried eclipse (with a vim keymap plugin). Now, eclipse kind of sucks in many ways, but one thing it definitely has is the ability to refactor code. Something like variable rename over multiple files that would have me concocting a sed-ish script for vi, or doing query-search-replace, next,next,next,skip,next in emacs was literally two keystrokes and bang, done. Forget about actually moving language constructs from one file to another or to another package, etc. And the UI for it was very clear: inplace modification, and popping up a warning about where the refactoring would break, if it would.

And then we have modern UI. Eclipse isn't even very pretty (or modern) by modern standards, but it still highlights properties of code (warnings, unused variables, etc) by actually drawing symbols or boxes on the screen rather than attempting to wrangle indications into the world of VT220 terminals. Among other things...

One could put this all making up for deficiencies in Java, I suppose. Except, again, if one is honest, one realizes that this kind of thing is highly useful no matter what language you're developing in...

Neither Vim or Emacs has very good support for any of this at all. There are examples of add-ons that do it kinda-sorta, but they aren't very good. Emacs is better than vim at this, of course[2], but it's still not very common for language support to get to this level. It's not a standard thing, and it should be. Such is history, and the pragmatics are writing a substantial piece of software inside another piece of software in a crappy language :-). Emacs still crappier (and slower) than eclipse at just highlighting syntax (and barely does the highlighting of semantics).

Emacs & vim have real issues as code editors because they are stuck in the past -- and not the recent past either. Vi was never meant to be anything but a text editor, for good or ill. And Emacs... well emacs lives in it's own little world reminiscent of a half-baked lisp machine crossed with a VT220 terminal. And while the capability is there somewhat to do the above, there is a lot of historical baggage and historical code to overcome.

So, is eclipse my editor of choice now? :-) :-P Well, no (except for java code). :-) It's a shitty editor, it's slow to start, it crashes or gets into a funky state too often, and it's really a Java-editor, not a generic text/code/anything editor (although it tries to be, and fails). But I think eclipse and tools like it show clearly "what is missing" in an old-school editor like vim/emacs, or indeed any new editor which uses either of these as a model.

[1] I also think key-chording, which emacs uses (and others influenced by emacs use) is actively physically harmful.

[2] A better scripting language helps. I'm sure emacs is somewhat passable at refactoring lisp code. :-)