Answering this as the author of the guidelines document (link in my signature).
Riria wrote: ↑10 months ago
I'm assuming the rule that a submission isn't allowed to contain anything except the card also bars CR excerpts, right?
Yes. Anything that isn't the text card and the optional render, and a CR excerpt isn't either of those, so it's forbidden unless explicitly required by the round challenges.
So, for example, if a card has a new permanent subtype with mechanical baggage, the reminder text should comprehensively communicate that baggage?
Again, yes. Reminder text is essentially the only means that you have to communicate rules baggage under ordinary circumstances, that is, again, unless required by the round challenges.
but the missing information is information that would in fact be missing if that were an official card printed by Wizards
The only thing that falls into this category is reminder text if it doesn't fit on the card on a rare or mythic. Nothing else comes to mind to which this would apply.
Follow-up question:
If a hypothetical card does not directly communicate enough about how it works within its own textbox to be intuitively used correctly, (above quote), what categories would this deduct points from? (Imagine, for example, that planeswalkers didn't exist in the official game, someone made a custom card that introduced the concept of the planeswalker card type, and the custom card did not indicate anywhere in its text that creatures can attack it)
I can see deducting points in the following areas:
Elegance - Mainly because of comprehension complexity, of if you prefer, because players can't understand how it works. Also, if it's due to lack of room, it means that the text is quite long, and that's another strike against it in this area.
Viability - Here three things count: color pie, rarity, rules. This would fall into the latter, as it definitely sounds like a rules problem. Not in the sense that the rules don't support what you're trying to do, but that they aren't either already known or explained directly on the card (usually via reminder text).
Balance - Here I'm thinking as a judge. If the players can't understand what the card does, the judge probably won't either. So the problem is: as a judge, how can you judge the balance of a card that you don't fully understand how it's supposed to work? You just can't. I have found myself in that situation as a judge before, and let me you, it's not easy at all. If I, as the judge, don't understand how a card works, I also can't understand how it interacts in all the different formats: limited, competitive constructed (Standard, Pioneer, Modern, in rare cases Legacy or Vintage), casual constructed, and multiplayer (including 2HG and Commander).
Main Challenge and Subchallenges - If the card is so incomprehensible that a judge can't even understand whether or not it fits the round challenges.
I don't think Appeal and Flavor would be affected. Uniqueness will probably be either very high, because it's a thing that has never existed before, or very low, justified by saying that a thing that has never been done before is useless anyway if you can't understand it. Quality would probably depend on the specific proposed wording.
Again, the host can create challenges that deviate by the guidelines, but that's not supposed to be the norm anyway. When it happens, the host has the final say on the challenges they created.
Speaking as a competitor instead, this is one of the reasons I asked the clarification above. I can't see how reminder text for battles in general and the battle subtype, plus rules text, plus reminder text, can all fit in the battle frame that we've seen in MOM. If the battle card has a new subtype, the reminder text will have to say what a battle type says: who's the protector and what happens when the last defense counter is removed (which is NOT defined anywhere in the rules as "defeating" the battle, by the way). If you want the most room available, the easiest solution would probably be to make a battle with no subtypes (and thus no extra rules baggage), and possibly making it rare or mythic so that you can drop reminder text if there is no room for it. In that case, you're getting rid of reminder text entirely and you'd probably gain the room to add some flavor text, and even then you'd probably have to keep it short. Consider for example that the only battle card in MOM with no reminder text is
Invasion of Alara, which is a rare. Even
Invasion of Tarkir has it, and it's a mythic.