What does The Joker fear?

[Ood Omega]

Nothing. The Scarecrow sprayed him with fear gas during Knightfall and it had no effect. Actually, he liked it.

Maybe that's more accurate -- it's not necessarily true that the Joker fears nothing, but he's so insane that he doesn't react to fear the way any sane person would. He embraces it.

-D

Basically this, he laughed off Scarecrow's fear toxin.

Fear is also a basic rationalization the brain uses to keep the body safe. There have been RL cases where mentally ill people completely lack this function. To be clear that's not the same as adrenaline junkies who use it to push themselves to achieve certain feats.

[LegoLawyer]

I'll second what someone else said earlier...boredom.

He's the extreme version of a kid who just can't sit still.

[Lancerman]

Reality.

He hated the real world and how it works so much that he lives a fantasy life with his own laws and philsophies.

[Nite-Wing]

Batman can laugh off the fear toxin as well that doesn't mean he doesn't have fears.

[Mr. Holmes]

Basically this, he laughed off Scarecrow's fear toxin.

Just because he laughs, doesn't mean there wasn't fear.

As for the thread topic, I say Lancerman nailed it.