This is an interesting thread. Thank you, Pen.
To present a very subjective range: I can and have done a reading just using scraps of paper with suit and number indicators on them, like Kng C for King of Cups, etc. There's no art to that whatever, and basically I'm just reading the inside of my head.
But it is not by any stretch the worst. Sometimes the art is offensive to me personally--such as (I'm making this up, but it's not too long a stretch to the real thing) Strength depicted as a tank rolling over a person and depicted in such a way that the viewer is obviously supposed to cheer for the tank. In short, the art contradicts everything I would have attributed to the virtue of Strength. In this case the art actually interferes with the tarot and I'd find it hard to read with such a deck.
Occasionally the execution of an image, even though the image is essentially benign or even joyful, will affect me in a similar way. The Woman of Trees in The Shining Tribe is an example. She's beautiful. And she makes me itch. Sometimes a whole deck is mildly to moderately irritating--body proportions are way off, the colors look like they've been cut out of paper and pasted on, etc. I can read with these decks, but they start out in my outer circle and they never advance.
On the other hand, sometimes the art is very good: the design is pleasing, the colors interesting, the figures elegant, etc. But each card looks like the next, sometimes to the point that unless the cards were numbered and titled, one cannot not tell which card is which. And I'm back to reading my own head again.
The decks I appreciate the most, the ones that I usually have within easy reach are those in which the art and the tarot reinforce each other--which they can do in so many different ways. An example: I don't really like adding meta-systems such as the Kabbala to the tarot. Yet I learned the Kabbala because of Navigators of the Mystic Seas. I felt like I had to learn that system to understand the art, the how and why of those images even though I already understood what they were depicting in reference to the tarot.
Ironwing is another deck in which the art and the tarot intersect very richly and on more than one level. The artist does a lot of metal working in addition to her paintings and drawings. Her "fool's journey" is that of an apprentice craftswoman who is learning to be a smith. Every image is either a depiction of a work of art or one that describes how to become a craftsman. And she is very knowledgeable regarding the fauna and flora of her environment. And she knows tarot through and through. So you get layers of meaning in each card.
Outside the range: Pixie. I've always thought of her as an illustrator. An artist, of course, but an illustration is art tweaked in a certain way. You try to catch the essentials of a scene but you don't want to put in too much lest you contradict the text or whatever is being illustrated. So the scenes she created have the essentials but leave a lot of things for the reader to fill in. That, to me, seems like a pretty good place to start.