The main thing for me is split panes, but I don't primarily use an editor like Emacs that can do split panes, and I never bothered to learn tmux (iTerm does have neat tmux integration though)
Ah right, split panes are pretty nifty. Very easy to do in Emacs and convenient too. When I'm using Terminal.app, I just use two windows and place them side-by-side if needed. But most of the time a single windows suffice for the small things I'm doing.