Nowadays, we're a bit afraid to talk about differences in quality. Everybody's a winner, everybody's beautiful, we all have different intelligences but they're all equal, everybody can do art and the main thing is intention. We're no longer supposed to say: that's good quality, that's bad quality.
But if we're honest, there ARE differences. There are sloppy decks just like there are charlatan artists, shoddy books and cheap music.
A good deck for me is a work of quality even if it's not my taste or won't read for me. A good tarot deck is based on thorough knowledge of what tarot is. The deck doesn't have to be filled with Hebrew letters, runes and esoteric symbols to show its foundation - sometimes these are merely exterior trappings that pretend erudition but are nothing but a facade. And a deck can be deceptively sparse in symbolism but allow us to find the symbolism in ourselves.
So there must be an understanding of the tarot, its structure, its history, its development.
And there has to be artistic knowledge. (Often, two people will work together or more - one brings with him the tarot expertise, the other a skilled hand and eye.) There are quality criteria in art, I won't go into that now... but if everybody was an artist, there'd be no need for art schools. No matter which technique an artist employs, he has to master it - water color or computer graphics. Art doesn't produce itself.
Within the magic triangle between tarot "as such" with its messages, structure, and archetypes - the artist/creator who translates tarot into his own visual language - and the reader who tries to read this language and connect it to his own life - within this triangle the messages have to run smoothly. A deck can be a great work of art but if it doesn't convey the idea of a card to the reader because it's symbolism is purely personal, it's not a good tarot deck.
From quality point of view, I think there is little doubt that Thoth is among the best there can be. It's based on a thorough understanding of a huge body of esoteric knowledge, and not only reproducing it but adding to it. The art is interesting, unique, well done (even though there are some technical problems with the human figure - the use of projective geometry and the quality of the color work are outstanding), and as complex as the visual language it - it IS readable.
For many people, not for all (there is a hilarious grumpy lady on Youtube claiming the Toth is "seriously fugly" and unreadable :-D), but for very many people the Thoth SPEAKS with its images. It conveys in abstract images, in form and color, feelings, associations, situations, moral dilemmas and evokes archetypes most strongly.
You can't do much better than that.