I love that the sliding meter shows tick marks for your previous rankings of other teas - the UI reflects that your judgement of a particular tea is relative to your other experiences.
I'm imaging a UI which asks you to pick a favorite among the item you are viewing and one similar item. You could stop there, or ask repeatedly with new comparison items until the viewed item's position on the absolute scale is unambiguous. The user could provide some rating data with just a single binary decision, but some ajax-y fade out/in of another pair could enable further ratings if they desired.