I won't say I'm never _ever_ going to get this or that kind of deck, because there may always be exceptions to the rule... but so far, I really haven't been interested in any unicorn or dragon deck. In fact, any deck that is themed like that, be it unicorns, cats, dogs, bunnies, aliens, mermaids or whatever... they just seem like novelties to me. Not to say they can't be cute and all, but they don't work for me. I'm not interested enough in whatever theme creature they feature to find them worth buying.
On the other hand, I adore the old Alice in Wonderland deck which is a pure RWS clone with Alice-inspired imagery, and I'm sure many would see that one as nothing but a novelty...
I guess it comes down to personal taste. And I do have two vampire decks. Vampires are my favorite monster. But I still don't like every vampire deck out there. There has to be something more than just "hey look, vampires!" to appeal to me. Great art perhaps, atmosphere, new take on card meanings, something different.
Edit: good art makes me forgive almost any boring theme. Interesting takes on card meanings/themes can make me forgive art that's not perfect. I mostly decide on a deck-by-deck basis, I think.