This won't still ever cover all readers out there and it only gives an indication of what's the flavour in reading of that era.
It doesn't have to. It can't. As they say there is always exception to the rule. If it covers the majority of the readers out there it's enough. On top of that people who are into tarot are one of the most diversified group of people of all! ATF thought me a lot in this regard. So finding a common ground is even harder. But this doesn't mean we shouldn't try
I'd like to bring my 2 cents contribution but I am totally clueless. Surely I agree with what have been discussed above and the validity of statements and points emphasized (1,2, etc).
But I am getting stuck because I noticed that some decks that a few years ago I found unreadable (more or less) I find completely readable now. That means there is a huge personal factor as well as one's evolution as a reader. That surely complicates finding a 'rule'. To find a 'rule' that would apply to most I think we have to leave out ('rule out', ha, ha) ALL the personal factors that makes a deck readable.
What remains then is what has been already stated in the previous posts:
- to visibly follow a system (thoth/rws/marseille) (JadePixie) otherwise I couldn't consider it a tarot deck but an oracle
- to has symbolic/semiotic visual clues (JadePixie)
- the images on the cards to evoke a reaction in people's imagination (even if they don't like their artistic style) (MandMaud) The card has to be visually expressive (and impressive! ha, ha)(I get bored easily with those deck that fail to impress me)
- the cards has to work well with each other, i.e., it's easy to see the dynamic, direction, focus, patterns of a reading. (MandMaud) (I'd add the uniformity of the artwork and fluidity but that could be a personal preference only)
That's a LOT to ask from a deck made of 78(!) cards! I believe any deck that complains with these requirements would be well readable for most people. Or would it?!
ETA I am one of those readers who mainly read for themselves