Also seconding. Vanilla emacs is already extremely powerful, and if you're doing real work, you'll end up having to use hosts that don't have all of your wonderful enhancements installed. It's good to know vanilla emacs.
Beyond that, add language modes for anything you need that's not already included.
If you use 'git', 'magit' is very nice.
Beyond that, helm is interesting, though very large and kind of mind-blowing.
Recommend these in your ~/.emacs.d/init.el, to avoid being that guy:
(setq-default show-trailing-whitespace 't)
(setq-default indicate-empty-lines 't)
; try not to use tab characters ever when formatting code
(setq-default indent-tabs-mode nil)
(setq-default require-final-newline 'ask)
(setq-default mode-require-final-newline 'ask)
Beyond that, add language modes for anything you need that's not already included.
If you use 'git', 'magit' is very nice.
Beyond that, helm is interesting, though very large and kind of mind-blowing.
Recommend these in your ~/.emacs.d/init.el, to avoid being that guy: