1- Is it wordy? Is the effect complex?
Nothing turns me off to a card if it fills the entire text area with a bunch of specific clauses. I personally would like to try to keep effects vague.
2- Is it original?
We've had so many card ideas submitted. Despite that, we still get a flow of new ideas, be they of original design or a twist on a pre-existing idea. If the idea gets approval, it should be bring something new to the table instead of reusing in-game mechanics.
3- Can this card be abused to easily with current popular/obvious mechanics.
Due to lack of cards that do make it into the game, card ideas should be made with the in-game cards in mind since this is the card pool they will be interacting with.
One of the first things to consider if it's a creature is Fractal. Can this creature cheap and easily fractalled? Does it have a trigger/ETB effect that would be troublesome en masse?
Aside from that, for other cheap things, it's kind of important that how easily can it be added to a Rainbow deck. Some effects aren't used as a general utility spell and would probably like to be seen in a more versatile manner...
4- Is it worth playing?
On the opposite end of the spectrum of being abused is if it worth using at all. When comparing it to the current card pool, your ideas shouldn't overshadow current ones. Nor should they be overshadowed by current ideas. And there should be some enticement to actually use the ability in itself.
There's no point in making an idea if there is no one who would use it.
5- Is this card in the right element?
One of the things I do judge on is thematics. Yes, a card may be element-shifted to prevent abuse or to make it more viable with another combo, but did it do so by sacrificing theme?
Honestly, theme plays a very minor role in card design and it is a pain to find an appropriate one for your mechanic if you didn't base the effect on the theme. But it is still part of card design.
6- How is the name?
Very little to do with the mechanics, but the entirety of card design is not the mechanic. As stated above, theme is part of it. And a theme is only conveyed well by a proper name.
We must remember that Elements is a fantasy-style CCG, so anything too modern or science-y immediately sounds out of place. While some names are technically correct, I feel there is some lenience needed in naming it in order to get things to fit as long as the theme isn't stretched too far to make it relevant.
Also, Elements is a relatively simple game. As such, I feel that the naming of cards should be kept with brevity in mind. Something short that delivers impact without sacrificing the theme, but still descriptive enough to convey the theme.