I'd never taken a really close look at his paintings before, just had the general impression that they were pretty weird. Well, a close-up look confirms it: they are (and by extension he probably was) not just pretty weird, but actually extremely weird.
Here's the Bosch enigma to me. He had an incredibly broad and prolific imagination, but spent his life working only within one consistent and narrow theme.
Everyone agrees Bosch was a terrific draftsman, but after that opinions vary wildly. Was he a simple commercial craftsman who discovered something that sold and stuck with it? Was he obsessed with spreading the word and reinforcing Christian dogma regarding the wages of sin? Was he a closet subversive, undermining that dogma via over-the-top lampooning of the theme? His work is strange enough that the wildest interpretation of motivation is probably tame by comparison. And that he painted these in the 15th century... hard to believe.
A look at a couple close ups of painting details: