When programming in Emacs, I found that the single most useful feature was dabbrev-expand (and perhaps the closely related hippie-expand), which takes a look at what you're currently typing and tries to complete the word by analyzing the text in your other open buffers. I mapped the command to that silly little key above the tab key and used it all the time.

I later switched to vim (mostly for the sake of trying a new coding environment) and this is the feature I miss most. The autocomplete in vim... well it doesn't suck but it's not really the same.

Oh, I almost forgot, check out dotfiles.com for a collection of useful init files and macros. I hope you realize that from now on your life will be a quest for the ultimate .emacs ... :)