I second wedrifid's recommendation for evil-mode. (I also use nonstandard bindings ;w ;b ;k for save, switch buffer, kill buffer respectively, and these are a lot faster than the equivalent ex-mode commands would be.) I also use auto-complete-mode just about everywhere; I don't have semantic completion (it's on my todo list), but it's very useful anyway. I like reading camelCase but not typing it, and often I can just type camelca<tab> to get camelCase. ido-mode is also a massive win.

On a lower level, binding F5 to compile has been a great improvement for me over my old "move mouse to terminal window, press up-enter" workflow. More recently I got further improvements by rebinding F5 in haxe mode to "find the makefile-equivalent higher in the directory tree, then compile" instead of using "M-x cd" in every buffer.

I haven't found org-mode as useful as others report. I like its structure, and I've written presentations in it; but my emacs start up buffer is an org-mode todo list that I haven't updated in over a year. This may be a more general problem with myself and todo lists. (I have found that writing them on paper is helpful.)