For Challenge # 083. I know flip cards are unlikely to come back, at least for a long time, but this just seemed like the simplest way to execute the idea. (My first thought was to use a counter to express whether the creature was on or off. That was obviously over-wordy.)

Keywords can be tricky. Sometimes they're hard to justify, especially when they take up such a small amount of mind space. In theory, you could just write this ability out on 15 cards... but I do like how the keyword saves you space on the flipped side, giving you the ability to add more stuff.

the problem is flipping too often. if this would appear on a lot of cards, imagine flipping them all after every single spelled cast by every player. also these are triggers, so they go on the stack before any of them resolves. would be worse on mtgo. if every 30 seconds you're interrupted by a stack of triggers.

It's true. I wouldn't put them on a lot of cards... just a smattering. But the problem of "That's a lot of triggers" is the similar to the problem that Izzet faces when it has a bunch of triggers of sorceries and instants. Admittedly, 'every spell' is much more common than 'sorceries and instants you control'.

That said, it should probably be as common as tapping lands to play spells, then untapping all your lands on each of your turns. Since the action is simple (it isn't asking you to move counters around, or shuffle your library or anything. Just pick up one pile of cards and flip them, and maybe do the same thing to another pile of cards) and all Abeyance cards do the same thing (If this was team Izzet, you might be allocating damage, untapping a creature, giving another creature a bonus to power, and drawing a card when you play one instant, for example) it should only frustrated a certain subset of players. How large that subset is, though, would probably require playtesting.