Sly means that a card gets played automatically if you discard it, so this is a cool option to pick up with something like a Dagger Throw to get going in early game. Nice that you can play it (even if it's not for a huge number) even if you don't draw it with a way to discard it. That said, ironically the existence of a mechanic like Sly means there's extra downside to polluting your deck with low-output cards, and this is one of those.
