Probably tmux integration, but the speed decrease against Terminal doesn’t warrant the additional features for me at least.

A trick I recently learned is to add iTerm 2 to the “Developer Tools” section of the Security prefpane - it makes it fly. Similarly if you’ve ported your config through several versions of iTerm, ensure that GPU-accelerated rendering is on.

It’s no stretch to say that iTerm 2 is a major one of the reasons I cannot stand desktop Linux (or Windows, even with the new terminal there).

For me, it's the little things such as ability to undo accidental terminal tab closing with cmd+z, restoring sessions after restart so I don't need to reopen a bunch of tabs and cd manually, ability to show timestamp for each command run, etc.