I mean "unplayable in limited". As a general rule, anything that doesn't either win you the game, negate one or more of your opponents' cards, or get you more cards is probably not worth playing. Common examples of cards that don't fit include creatures with less than 2 power, less than 3 toughness, and no major abilities, like
Sanctuary Cat and
Tree Monkey, lifegain like
Elixir of Immortality and
Staff of the Death Magus, stalling effects like
Fog and
Silence, and cards like
Spellbook and
Witchbane Orb that just don't actually do anything that matters in most games.
Other examples of cards that are bad in limited include most auras (outside of Theros), such as
Holy Strength and
Magefire Wings, because they open you up to being 2 for 1'd if you opponent has removal, as well as expensive cards that don't win you the game, like
Worldfire and
Warp World.
Um... I just have to disagree with some things. 1 drops like
Sanctuary Cat and
Tree Monkey are fringe playable to fill out mana curves, and as chump-blockers. They can also tick away damage when your opponent has no blockers.
Elixir and the Staffs are actually pretty good in limited. They suck in constructed, because you've just plain old got better options most of the time, but lifegain is stronger than normal in limited, and elixir can provide pseudo-recursion of some of your better cards, as well as protect you from milling yourself out if you are using dredge type tricks or card draw in a limited game that drags on for some reason (which they occasionally do, especially if both players have lots of removal). Fog and Silence aren't great, I'll admit, but within color restrictions, they aren't really _that_ bad.
I'll give you Spellbook and Witchbane Orb, sometimes I was just throwing stuff at the wall.
Magefire Wings was my only non-creature option for RU besides Izzet Charm to complete a cycle of such.
I'm not sure Holy Strength is _that_ bad, given that it's a 1 drop, but I think I might take it out. Although I will point out that not only Theros auras are playable. There are some that are cantrips, re-usable (like Rancor), or grant removal resistance (like the totems), which are probably playable.
I agree on Worldfire and Warp World though.
Edit: Wait, no, the lifegain stuff was bad advice I got from a WotC mothership article! Ignore my stupidity there! It's not good without doing something besides the lifegain. **** the Staffs.
Edit2: took out Spellbook, Witchbane, the Staffs, Holy Strength, Worldfire, and Warp World, added that cycle of enemy color protection dual ally color creatures.
Edit3: Checked again, maybe I should replace Magefire Wings with
Swerve?