Also, TDKR was the first work in which Superman and Batman represented two supposedly irreconcilable approaches to crimefighting, with Clark falling on the idealistic, minimally violent, rule-following side and Bruce on the cynical, brutally violent (short of killing), screw-the-rules side. It was also the first (I think) to make Batman's motivation more disturbing and (even though Miller himself would later deny this) pathological, in that it's not just his parents' death, not just Gotham's continued decay, but an inner Bat-voice that pushes him back into crimefighting ("You know it in your soul...for I am your soul...").

In short: I agree that grimdark Batman (the Modern Age one, not the original Golden Age, gun-wielding "Death...to Doctor Death" one) has its origin in TDKR.