it all comes down to how much you enjoy it. sure you can suggest criteria to look for, sweetness, creaminess, foam, spices, flavor, etc. but i don't think most people will agree just where on each of those scales a "great root beer" will fall. it is subjective, just as any other food or beverage tasting will be.
the stuff suggested above is great for helping describe what you do or don't like about a soda though. i just don't think it should get that snooty and technical.
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