Well, here's how I go for decks based on various criterion:
1. Artwork - The artwork needs to be rich and deep. Even black and white decks can have incredibly rich artwork (Light and Shadow Tarot or Diary of a Broken Soul), and possess a hauntingly beautiful quality. Decks where the art just seems to be done by a retarded chipmunks and a box of crayolas is so not my cuppa tea. Gaudy colors turn me off as well. Decks which resemble random splashes of colors on them really turn me off.
2. Death, Devil, 3 9 and 10 of Swords, and all the 5's - They gotta be intense, they gotta have an impact. They so need to be stunning, but not fluffy looking. I don't want titles like 'Transformation' or 'Super huge change' or 'Bunny no likey this card'.
3. Human Figures - They have got to be well drawn and aesthetically appealing. No, they don't need to be Barbie n Ken, but a level of aesthetic value is required. Some decks I own don't really contain classically beautiful human figures (World Spirit, Thoth, RWS, etc), but they possess a quality to them that makes them lovely and aesthetic. I'm not at all opposed to nudity, and decks where the Star is 'clothed', I throw away. Decks that are 'censored' totally turn me off.
4. Expressive Minors - Minors are more often than not, neglected by the deck creators when creating a deck as many a times a whole lotta attention is focused solely on the majors. If a deck has minors that look 'abandoned', or where they look like they were rejected by a 'caste system' of sorts... that deck is so not for me.
ETA: They don't need to be illustrated in order to be expressive. The Thoth minor pips are a gazillion times way more expressive than RWS styled deck minors. There I said it!
5. Readable Courts - Court cards just have to be readable. They gotta be designed in a manner where you get a feel of their energies and characteristics. I like to see my Pages as women (or Princesses), I like seeing my Queens as lovely, I like seeing my Knights in shining armor, and The Kings have gotta look grand. They should be richly dressed, beautifully illustrated, their faces should be expressive, contain features that describe their personality (a Roman nose for the King of Swords, a cleft chin for the Knight of Wands for example).
6. Symbolism - They have gotta be rich in symbolism. And for me, some symbols absolutely 'must' be included within a card, for without them, the card seems to loose it's value (like the infinity symbol on top of the Magician and Strength maiden). Also, in the minors, I like the standard Wands, Cups, Swords, Pentacles. I can handle using elements, or even Scepters, Coins, and stuff, but please don't gimme decks that have stuff like, Bowling Balls, Clouds, Dildo's, and Lipsticks for the minors.
ETA: Gimme lots of Astro/Qabbala/Esoteric symbolism - I Love it, but at the same time, it should be aesthetic!
However... despite all this, I sometimes am attracted to decks which totally go against all my "rules", cause they just seem to 'speak' to me. An example would be the Mystic Faerie Tarot... it's so damn cute
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Hope this was helpful
Love and Blessings