Here's an example. The other day I needed to number the lines of some code examples in notes I was writing up. Not a built-in editor feature, but a few minutes' hacking in the One True Editor yielded:

(defun my-number-region (beg end)
  (interactive "r")
  (save-excursion
    (let ((end (progn (goto-char end) (beginning-of-line 2) (point-mar
+ker))))
      (goto-char beg)
      (beginning-of-line)
      (loop for i = 1 then (1+ i)
     while (< (point) (marker-position end))
     do (progn
          (insert (format "%-6d" i))
          (beginning-of-line 2))))))
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Being able to do this sort of simple customization without knowing about "plugin architectures" and what-not is what I think Emacs got fundamentally right, and is why I never have the patience for Eclipse.