So Ask Jeeves told me...

... That in Greek mythology there's a river called Lethe. It's waters caused forgetfulness and was on the banks of that other Underworld river, the Styx. If you've never heard of that one, then you are exceptionally lucky, cos I remember never studying anything other than Greek mythology at primary school, and the Styx is basically just a river with a ferry man to the Underworld. So all the dead people would gather at the banks of the Styx to pay the ferry man Charon to take them across the river. If they didn't have the cash (their relatives were supposed to bury it with them) then they just had to wander around aimlessly forever.

Anyway, so if you drank from the river, you forgot your past, which ties in quite neatly with what Willow used it for. However, it was considered a bit of a bad thing, as once you drank from it you got left with jack to reminisce about for all eternity.