How can anybody say that the art style defines Zelda when they change the art style with pretty much every 3D entry, and basically every 3D entry past OoT/MM has received some level of criticism for its choice in art direction? Wind Waker was cited as too childish, Twilight Princess was cited as too dark and dreary, and Skyward Sword basically was treated as "if I didn't like the Wind Waker art style, why would I like this".
There is no defined way of looking at Zelda's art style because there is no defined Zelda art style. There is a relatively defined gameplay style, and as somebody else stated, as long as that is present, the game is still Zelda, which is exactly why we can ultimately excuse the extreme inconsistency in presentation from entry to entry.
