There's one huge flaw with the original post, and that is the fact that these "unwritten rules" were not written by Zanz, they were written by a player.
How can we know that "No cards that affect the "graveyard" is an "unwritten rule"? We don't know if there will be a graveyard or not. Even Zanz might not know that at the moment. The fact that something is missing from the game currently, does not mean that there is some "unwritten rule" that says it will never happen. It's like me saying there is an unwritten rule against having pigs in Elements because there are no pigs in Elements currently.
Basically original poster (who is not a developer) is telling others what ideas are not going to happen because of some "unwritten rules" that he (not the developers) made up. And the problem is that these theories were written to sound like the law.
I was thinking about starting a "Good Ideas vs Not So Good Ideas" -thread on this section where we might list some of these things that would help players to design better cards. Some of the things listed on this thread might go under "Not So Good Ideas" but there are definitely no "unwritten rules" that say they could never happen.
So yeah.. Wording on this thread needs to drastically change. Otherwise this thread only limits creativity which is opposite of what this subsection is supposed to be about.
I'll further break down these points:
No cards that affect your opponents hand directly
I don't get this one. What's wrong with a card that for example randomly discards one card from your opponents hand? Nothing in Elements suggests that it could be done.
No cards that modify either players deck directly (except to put a card directly into a specific position in the deck, thus far only on top).
I don't get this one either. There's nothing that would suggest Zanz is against this kind of mechanic, and it would be relatively easy to pull off.
No cards that take control of an opponent's creature. This is a staple of ccgs, and the fact that it doesn't already exist says that Zanz prolly doesn't want it to.
If by "taking control" you mean that the creature stays on your opponents side, yet you control it, then yes this could be an "unwritten rule" because it wouldn't make much sense gameplay wise.
No cards that have multiple non-"all", non-"random" targets. The targeting mechanism clearly is designed to handle one chosen target at a time.
I kind of agree with this one. Any card that has "click three enemy targets" type of ability would be a bad choice, but effect could easily be "3 random targets..". Absolutely nothing suggest that there's an unwritten rule against this.
No cards that kill a creature without taking it's HP into account on some level. (Aflatoxin used to do this until I pointed out that it was the only card that did, and Zanz changed it to be like Poison instead. Shockwave kind of does this, but it requires 2 elements and another standing status effect to work.)
Mutation "kills" any creature regardless of HP. Nothing suggests that "instant kill cards" are off-limits because we already have it. Zanz might have nerfed Aflatoxin simply because it was overpowered.
No cards that require you to target a quanta pool. This is probably flexible if the idea is cool enough.
Huh? Um.. why?
No cards that affect the "graveyard" (to borrow a M:tG term). The game doesn't seem to have any tracking for cards that have left play, so that's a no-no.
It's not a no-no until Zanz says it's a no-no. Again, this kind of mechanic could be done and nothing suggests that there is an "unwritten rule" against it.