I agree this feature would make for an even more enlightening experience. Color/shade the highlighting as a function of its total lifetime on the screen. Words that are typed and deleted immediately after are the darkest yellow, words that live longer (like until the 2nd editing pass) are a lighter yellow, words that survive to the final remain white.
Now a killer feature would be highlighting text that will be deleted immediately, not just in retrospect. =)
I guess, some super-smart NLP algorithm could be developed to make that possible—once enough data on what gets eventually deleted is collected.