You know, it's funny. I said the exact same thing you did, then a friend convinced me to try iTerm2 about a year ago. I still couldn't put my finger on what's better about it, exactly, but I've never considered going back after the first day. It just feels right.

It's probably easier to try than it is to find out why to try it...

Panes and saved pane layouts are handy; I generally want to move all my terminals in a group and they're great for that.

It also has great tmux support built into the client.

The only one that keeps me using it is mouse support. Terminal.app doesn't actually send any mouse events.