The killer feature for me is Edit>Selection Respects Soft Boundaries, which lets you copy text from inside windows defined INSIDE the terminal - like tmux or emacs splits - where iTerm figures out that, e.g., a pipe character is a window boundary.

Two more:

2) if you accidentally close a tab/window, you have a few seconds to hit ⌘z and the window will reappear as if you never closed it!

3) Minimum color contrast. If your terminal color scheme and what you're running's color scheme interact poorly to create something that would be unreadable, iterm notices and you can have it override those colors to be something of higher contrast, automatically.

But that's just my killer features. iTerm is like Word - it is a bloated monster with thousands of features. Nobody needs them all, but nobody agrees on which ones they need.