- Bafflingly, they split horizontally. I have a 4x4 pane setup, but my default is 2x1.

- Cmd-W closes the window, not the pane. (I get why they do this. I feel that it is wrong.)

- I can't send input to all panes, which is a pretty useful feature I probably use once a week.

- iTerm2 happily maps my panes to tmux. This is by itself perhaps a killer feature.

Thanks! I feel the same about "tmux vs screen": tmux panes are just so much easier to use and more flexible than in screen, even though screen can split windows).