I've always found == to be very useful when comparing undefined, because undefined == null. It's very rare that I want to treat undefined and null separately (unless one or the other means there was an error earlier).

That's the only reasonable use for ==, IMO.

I recently added my own syntax rule to JavaScript mode in Emacs to highlight lone { at the beginning of line (with or without trailing whitespaces). After your comment, I think about adding some more highlights for with, == and !=.

Are there any other hard and fast rules I should be aware of?

Are there ever. Read "JavaScript: The Good Parts" or watch the lectures by the same name on YouTube.