A great and brave post, Pound. Sorry to see that half the images have been removed. But it still reads well.
Here's something that no one has really touched on yet: symmetry, which actually relates to the broader issues of the history and science of playing cards. There are others who know far more than I do about how cards have evolved, but there are certain principles that are integral to what cards are today, and foremost among these is symmetry. Once upon a time, court cards were one-way and, as Don observed, this provided a tell to others when you turned the card right-side up in your hand. Often, back designs were one-way, too. The first trick I learned as a kid was if you simply turned a card around before returning it to a deck with a one-way design, you could easily find it. Discerning card players always insist on a symmetrical back design.
There are two kinds of symmetry in cards: 1) what happens when you draw something and hold a mirror to it, and 2) a more sophisticated interlaced symmetry, most commonly seen on court cards. Within category one, there are important variations. To my mind, the worst offenders literally draw a line and cut and paste the top to the bottom. Sorry, but that's what the Jaqk deck does, and I think that is the least pleasing form of symmetry for a deck of cards. The second variant of category one is the design that still passes the mirror test, but does so with a central design element. Here's a bold example, Bicycle Allwheel:
[http://www.bicyclecards.org/Bicycle/02_Allwheel/red.jpg]
But some of the best card back designs feature symmetry that does not pass the mirror test. Because this is what I am most familiar with, I will again cite Bicycle cards: Acorn, Chainless, Thistle, and Wheel #2. Cupid and Motor #1 accomplish this in a very subtle way. Mobile #1 is arguably the most freakishly spectacular.
I guess my message to prospective card designers is: be aware of all of the design elements of your deck, and, following on Don's advice, pay particular attention to those that are peculiar to playing cards. Know your history, which may well give you the freedom to boldly dismiss it and do something innovative and hugely successful.