I agree with Alan Storm: "because the patterns and metaphors used in Emacs may not align with you brain"
This is a very important factor. Different brains adapt differently to different interfaces.
Some of the main - and easily available - features I really love Emacs for, and I count as productivity enhancers:
1. "yank-pop" facility - every cut/copy is saved into a stack so you can later choose which to paste (don't know if vi / Vim has this but most Java IDEs don't)
2. the Ctrl-key navigation mapping - this allows you to navigate your file without moving your hands off to use the arrow keys. (key-binding in other editors helps of course)
3. available on almost every platform (true of vi/Vim too of course) - whether GUI- or text-based (Java IDEs are available on most platforms too but only in GUI mode, and are significantly larger and need to be installed separately whereas Emacs is generally more widely available - BSD / *nix / Linux / Mac systems
4. I prefer my editor to stay out of the way until I need it - Emacs' spartan display forces me to think before I type.
5. The basic navigation keys in Emacs are kind of universally available - on my Mac OS, I can use these keys in terminal, mac mail, etc.
Ultimately, if Emacs' philosophy appeals to you, you will put in the extra effort to learn it. And it will reward you.
