Okay, so for me the line is all about internal logic versus rule-breaking whimsy. Classic fantasy, think Tolkien or even Sanderson, builds this incredibly sturdy world with consistent rules—magic has a cost, history matters, the map is detailed. A crazy fantasy book often uses that classic structure as a trampoline. 'The Hitchhiker's Guide to the Galaxy' isn't technically fantasy but that spirit is there—it takes a premise and just bounces it into the absurd. In proper fantasy, something like 'Piranesi' feels crazy because the logic is so dreamlike and personal, not systemic.
What really defines it lately for me is tonal whiplash. A classic fantasy might have humor, but the crazy ones let the tone careen from deadly serious to utterly ridiculous within a scene, like in 'The Lies of Locke Lamora'. The world feels lived-in but also slightly unhinged, like the author is constantly asking 'what if?' and then sprinting down that path without looking back. It's less about creating a believable secondary world and more about using fantasy as a playground for wild ideas, even if it gives the reader mental whiplash. I live for that feeling.