The only feature that I find even remotely compelling is mouse support, but so few programs handle mouse input that it largely doesn't matter. People always mention split panes, but I've never had a desire to vertically split my terminal window. If I ever did have such a desire, I'd probably just use tmux. And yes, iTerm2 claims to have some form of tmux integration, but I believe that's basically just using iTerm2 split panes to represent tmux panes, and I don't get why that's any better than just using tmux directly.

So besides mouse support and split panes, what does iTerm2 offer that Terminal.app doesn't? Some of the answers I've heard in the past are "256 colors" and "color themes", both of which Terminal.app supports.