For most languages (however, notably not Java), Emacs can have great auto-completion. I personally really like auto-complete mode. It might take a little bit of effort to configure auto-complete mode to work very well with your favorite language, but I've found it well worth doing: it's far, far easier than learning some new editor and lets me keep using the rest of Emacs's great features in any language I happen to be using.
EDIT: Reading some other comments, there is Emacs-eclim that gives you Java IDE features inside Emacs. So forget what I said about Java: Emacs can now handle even Java well!
For dynamically typed languages, Emacs can essentially be as good as possible fairly easily. I've been using it for JavaScript and Racket lately, and it's been really great for both. For statically typed languages, there are often type-aware tools that make it as good as any IDE--for example, I've used TypeRex[1] for OCaml; there is also ENSIME[2] for Scala and similar tools for a bunch of other languages.
Just because Emacs is a great text editor does not mean it isn't a good IDE. It has some very good support for a whole bunch of languages. I'm under the impression that Java is an exception to this rule, but I really don't know--I haven't used Java in a couple of years and really hope I will never have to use it again.
So, all told, I think this oft-repeated advice to use an IDE instead of a text editor like Emacs is misguided: Emacs is much more an IDE than just a text editor. Perhaps it's accurate for Vim, which I've never used (and I wouldn't be surprised if Vim support IDE features well too), but it isn't anywhere near true for Emacs.
EDIT: Reading some other comments, there is Emacs-eclim that gives you Java IDE features inside Emacs. So forget what I said about Java: Emacs can now handle even Java well!
For dynamically typed languages, Emacs can essentially be as good as possible fairly easily. I've been using it for JavaScript and Racket lately, and it's been really great for both. For statically typed languages, there are often type-aware tools that make it as good as any IDE--for example, I've used TypeRex[1] for OCaml; there is also ENSIME[2] for Scala and similar tools for a bunch of other languages.
[1]: http://www.typerex.org/ [2]: https://github.com/aemoncannon/ensime
Just because Emacs is a great text editor does not mean it isn't a good IDE. It has some very good support for a whole bunch of languages. I'm under the impression that Java is an exception to this rule, but I really don't know--I haven't used Java in a couple of years and really hope I will never have to use it again.
So, all told, I think this oft-repeated advice to use an IDE instead of a text editor like Emacs is misguided: Emacs is much more an IDE than just a text editor. Perhaps it's accurate for Vim, which I've never used (and I wouldn't be surprised if Vim support IDE features well too), but it isn't anywhere near true for Emacs.