Would be really useful to have a 'Cancel' button for the posting editor, in case you changed your mind and didn't want to post it after all. Most other forums have them.

Which other forums. I have never seen such a button?

The Quick Edit does have a cancel button, the Full Edit has your browser's back button. If you're wanting to cancel after you've posted, though, that's trickier, since IPB does not save the old data anywhere.