this is sort of a personalized version of OT's "Card Design: An attempt at a comprehensive guide (A community effort I hope)"
http://elementscommunity.org/forum/index.php/topic,16655.0.html that has gone by the wayside as of late. it will contain my thoughts and rules/regulations for card creation used in the my personal card development process. hopefully it will help some in their card development.
(rules are ranked by their order in the process, not by their importance)
rule #1: do something new
the CIA forum has been around a while, and a ton of ideas have already been created. the purpose of the CIA forum, in my opinion at least, is the provide zanz with novel ideas that could improve the game. if you think making cards is fun but the idea you came up with isnt novel, it isnt worth submitting and taking up a spot in the polls, which are limited to 18 for each poll at the end of the month. if it has been suggested before, but the offering is just another way of doing it, or under another element, and/or with a different name, its time to stop and think of something else, or completely overhaul the card. there are few exceptions to this, but for the most part, i just start over and save the spot in the polls for someone who presents a novel card. however, if the card is in the game, and you think you can provide a twist to it that will be interesting and provide new gameplay mechanics, this is a valid exception that most people can follow. also when there have been previous attempts at a given card name (hydra comes to mind), don't bother, I'm sure people are tired of it. there are other ideas which have been attempted or suggested many times and the general public and/or zanz doesnt like them, if someone says "this has been suggested a million times" (card graveyard) or "zanz has said he will never do this" (card graveyard, too), stop the thread and thank those for letting you know.
rule #2: KISS- keep it simple, stupid.
we have all seen the cards that have a million stipulations crammed into the card text with abbreviations and varying degrees of clarity. dont do this. it looks bad and is usually hard to understand. some things are okay to be left off the card text(ie destruction of a bone wall is not equivalent to the destruction of any other permanent in relation to a given card mechanic based on the destruction of permanents by your opponent), but most aren't.
rule #3: art does matter
i spend a lot of my time searching through publicly available for use artworks, if i cannot find art for it, i will not post the idea. art not only gives people their first impression of your card idea, but it also gives a little help to zanz should he want to use the idea. personally i find it infuriating when someone posts in level 0 without including any card art, or even worse, no card image. there are tools provided that make this simple, and if i could learn to use the free image editing site pixlr, you can too. i wrote more about card art in the last page of OT's thread. using ms paint art was funny once, every time since has been insulting. do not steal art from artists just because you did a google images search and really really really really like it.
rule #4: respect the elements
certain elements have mechanics that are soundly theirs-
aether has creature immaterial, lobotomize
air has damage doubling
earth has boosting hp in large chunks and permanent immaterial
darkness has stealing (health, cards)
death has on-death effects
entropy has mutants
etc
if you even think you are borrowing a mechanic from another element, include the cost of that element in some way, be it the card cost (element of the card), card ability cost (cost per use), drain of quanta (costs 1 relevant quanta each turn while in play), or a creative interaction (such as more potent when you have some relevant quanta when played, its more effective, it doesn't even have to necessarily be quanta that it interacts with, there is no box, just get them to have synergy beyond taking the mechanic away from the element)- the latter is tough but if done elegantly can really be a boost to the card. when i cannot find a way to incorporate the borrowed element, i start over.
rule #5: balance the idea, reasonably
when i first started making cards, this was the hardest part. whats this skill worth? again, OT's post goes into this a little bit. however, you will never appease everyone, especially newer folks- who's responses you should weigh less than those who are established players/card designers, but when you reach the point where people are suggesting +/- 1 here or there, it generally should be good to go. some cards a bit trickier due to the existence of card such as fractal, mitosis or even supernova. people are usually quick to bring these up if overlooked, and adjustments will usually be necessitated, and in some cases, the card may just be scrapped entirely.
if something else comes to mind ill add it.