Simple ALL games are unique in their own way. Even if one game copies another game 99.9% that 0.1% makes that game unique from all other games.
Also WOW is a unique game, otherwise all MMORPG would do just as well as WOW. Sure it copies the conventions of other games (i.e. races, classes, monsters, spells, etc.) but when you combine it all you get WOW as opposed to Runescape. Creating a game is like cooking, you can have 10 different cakes which use the same core ingredients -- eggs, flour, water, yeast, sugar, etc. -- but depending upon how you mix these items you'll get a different result every time. WOW uses the same ingredients as other MMORPGs, but yet it mixes them in just the right amount to create a unique MMORPG experience.