The biggest "can't live without" for me (this goes for both VIM and Emacs) is the fact that this text editor is designed specifically WITHOUT a mouse. Yes, mouse support has been added, but I can do everything much, much faster sticking to the keyboard (which is weird, as that's where the "text" comes from) than being forced to:

  Edit >> Find... >> (type, type) >> Replace... >> (type, type) >> Replace All.

vs

  :%s@(type, type)@(type,type)@g

30 seconds and a whack of context switches compared to 3 seconds and none.

Those who prefer Notepad++/textpad/ultraedit/yada-yada are for the most part tethered to their mouse. I, for one, despise having to move WAY OVER THERE to get to my mouse (now how can I preview this post without the use of...D'OH!!!!).