Q: How did I not know about this? A: You never read the manual.
(A close friend of mine from college with whom I've worked professionally now and again, once suggested that the secret to our individual successes was that we were the ones willing to read the manuals. Obviously in this instance I failed.)
Oh, good lord, the number of times I've done M-x UP RETURN RETURN to
re-do query-replace on a new buffer, when I could have just done
C-x ESC ESC (repeat-complex-command).
Yeah, I realize that it's really only one fewer keystroke, but I can do that set of keys more efficiently. And I suspect that if I'm paying attention, I will find that I have far more opportunities to use it than just speeding up search and replace operations.
Of course, M-x list-command-history is a great complement to it, to
see what you have done recently. It will be interesting to play with—I
was honestly surprised at the contents when I tried it just now.