It was a lot more than that, especially when viewed in context of the time in came out. Aside from the exquisite Bolland art, it was the most detailed examination of the Bats/Joker dichotomy up to that point.
Plus it's the nature of the possible origin that helps sell it - If the Joker's origin is true, then, like Bruce Wayne, the Joker had the same sort of tragedy as Bruce did, the loss of a loved one, the proverbial "one bad day", but instead of using that to motivate him to hold on to life, it motivated him to embrace chaos. If, as is just as likely, the Joker's origin is false, then it adds a level of almost pathos to his insanity.