Just a little list of 10 things I feel are crucial to card design to be successful. These are aspects about a card idea you should ask every time you make it. This applies to card design in general, not just EtG.
1. Give it a purpose
This is probably the one that is ignored the most. This basically ties in the open-ended commandment you start with, with the others. The idea is that make every choice with good design in mind.
2. Make sure the theme fits
The theme of the card should fit the element and the card's mechanic, otherwise it is just weird looking junk.
3. Make it original
Not all parts need be original (especially card stats), but the more unlike other cards it is, with exception to purposeful similarity, the better perceived the card will likely be.
4. Make sure it belongs
This is sort of the same as 2., but this is more about the mechanics. A single mechanic should find itself in more than one faction/element, but probably shouldn't be unless this equality is necessary to the core game (like Pillars).
5. It should not break any part of the game.
While a lot of elements have notable synergies within themselves, these synergies should not be the best of their nature.
6. Promote Mutli-faction synergy
Not something that is too necessary, but keeping in mind this opens up a duo or trio to an option is a good idea. This basically is the opposite side of 5.
7. Make it balanced.
This can be a tad loose, based on other commandments, and also because no official balance method exists, and we are left with card cost theory threads, but make sure it just simply isn't a huge effect for a puny or nonexistent cost.
8. Keep it possible and manageable.
A good card idea does not need to be complicated, and complexity often draws developers away from it given the workload associated. Quite often, a well-designed card might actually take parts from various cards in its own way.
9. Give it nice art.
Nothing sells a card's appeal more than its art, just remember to not go overboard.
10. Make it cool!
With all the mechanical choices made, remember that while any card can go through the other 9 commandments, some would appeal more to the general public just because of the theme.
Feel free to yell why you think one or more of these is wrong.