What not to put in your Survivors' Cube

When putting together a Cube (for drafting or otherwise), you're playing the part of a game designer. You're not trying to "solve the puzzle", you're creating a puzzle for the player (who might also happen to be yourself) to solve. MaRo has written some great columns touching on this topic. We're not trying to create a situation where the player wins every time, we're trying to design a fun experience. The zombies need to be able to, occasionally, crest over the hill and see a group of exhausted and broken Survivors to eat.  Here are some things that will break the format, either instantly or only in large enough quantities, or will simply create "feels bad".

Useless Effects that will do nothing:
Due to the properties of Horde, a number of standard Magic effects do a whole load of nothing. While you'd still have a 2/1 creature, a Goblin Shortcutter will make the players feel like something was wasted. Just don't put them in the Cube. There are no shortage of vanilla creatures that will be effectively the same in the game but without the bad feelings.

• Creatures with evasion: The Horde doesn't block, so this is a moot point. Flying, landwalk, unblockable, they all just read "useless text here" when fighting the Horde. Some creatures have additional text that makes them worthwhile, but when in doubt, edit out. 
• Land Destruction: The Horde controls no lands, so a Stone Rain has no place, except to troll fellow Survivors. Some land destruction cards, like Maw of the Mire, also have another effect. These effectively become "sacrifice a land, do effect", and can make interesting includes.
• "Can't Block": Goblin Shortcutter and other "panic" cards do nothing, as the horde can't ever block.
• Artifact Destruction: most Horde decks don't have artifacts. Naturalize is fine since it also destroys enchantments, which the Horde DOES have. 
• Doom Blade: All (or at least most) of the Horde's creatures will be black. "Target non-black creature" is really just a dead card. 
• Trample: the Horde never blocks, so this is never relevant
• Reach: There are very, very few flying zombies for this to be useful against. Most Hordes will have none.
• Sorcery-speed bounce: a Silent Departure does almost nothing, as cards in the Horde's hand get cast for free the next turn anyway, and bouncing doesn't remove blockers because there are none.
• Discard: Mind Rot and similar effects do little, since the Horde only rarely has cards in its hand. The situation won't come up often enough for anyone to put this as one of their 23 cards in their deck.
• Infect: The Horde can only lose through running out of cards, so effects with poison counters do nothing.

One-hit KO cards:
Since the Horde is not at all like a deck for 1v1 duels, there are NUMEROUS cards that can single-handedly break the Horde. Don't put anything like this even in the same sleeve color as your Survivors' Cube. These cards can neuter the Horde in a single drop. You'll know them when you see them, because you'd say to yourself "Oh man, this would be GREAT against the Horde!". As a game designer that should mean "NOPE NOPE NOPE NOPE". Remember, fun, not victory. Here are just a few examples of broken cards not to give to the players.

• Moat: the Horde has no (or few) flying creatures, so this is pretty much an instant win. 
• Eldesh Norn: Most of the Horde's punch is in the form of 2/2 tokens, which this continuously eliminates.
• Platinum Angel: The players can't lose, and the Horde has no targeting ability to remove it.
• Mind Grind : The Horde has no lands, so this is an instant kill
• Win the game/Lose the game effects: As cool as Door to Nothingness is, these kind of effects are out of place fighting against the Horde.

Really strong effects to stay away from:
Some classes of effects are just really brutally effective against the Horde. They get big too fast, or are too efficient at shutting the Horde down with a single card.

• Protection from Black or from Zombies: these creatures are just too hard for the Horde to deal with.
• Lifelink: A creature with lifelink can single-handedly create victory for the Survivors. Not instantly, but the extra life round after round makes things too easy.
• Storm: with three or four players working together to drum up a big storm, it's even MORE broken than usual.
• Wight of Precinct Six: Creatures/effects that get buff based on the Horde's graveyard are just too strong, as it often has 10-30 creatures there.
• Lumberknot: cards with "when a creature dies, do X" are just brokenly powerful in this format, since zombies die (a lot) and you chump block (a lot).

Things to use only in small amounts or cautiously:
In putting together your Survivors' Cube, you have a finite "budget" of power to spend on cards. If you overspend, the Survivors will be too strong and the experience will lose tension and balance (and thus fun). Some effects are strong, but OK in small numbers since this is a Limited format. Cards with these effects should be enough to pull someone towards that color.

• Vigilance: In this format, vigilant  creatures are VERY good. 2/3 or bigger vigilant creature is a monster here. A Thraben Valiant, or Standing Troops, though, can be very fine includes.
• Life Gain: Too much life gain really can suck the fun out of this format. Make it rare and weak. Cards that can reliably generate 2 or more life a turn on a continuous basis should be avoided. 
• Graveyard Hate: Too much of this at too high a power level can really neuter the Horde's late game. Crypt Creeper good. Tormod's Crypt bad.
• "Saboteur" abilities: Stealer of Secrets, for example, is much better against an enemy that can't block. These effects must be rated very highly, and thus used only a little.
• Board Sweeps: Killing all the Horde's creatures is something that does need to be included, but not so much that it is likely that there is ALWAYS one on deck in someone's hand . Use sparingly.
• Cheap instant creature removal: Too much of this and players won't need to save it for that Grave Titan they know is coming eventually. This should feel like a precious resource. Make removal spells more costly (4 or 5 mana) so the players really have to sacrifice something to remove a good zombie. If you've got too many Smites in your cube, it really dampens the fun. 
• Counterspells: Stay away from cheap, efficient counters completely. Even Cancel. Spell Blast and Geist Snatch are good choices, since the Blue player needs to leave a good chunk of mana open. Two counters in your pool should be enough to pull strongly toward Blue in this format. 
• 6 or more toughness: anything that can take a Zombie Giant on the chin should be used sparingly.
• Cheap First Strike: First strike creatures are very valuable defenders. Don't make too many high-quality first-strikers a part of the Survivors' Cube. Warclamp Mastiff good. Porcelain Legionnaire bad.
• Fog: Giving the Survivors a turn without having to worry about the Horde's creatures is very powerful. Necessary, but don't go overboard with these.
• Cheap Deathtouch: a Typhoid Rats basically reads "kill the Horde's strongest creature for 1 black mana". Don't include too many. Maybe one Ambush Viper.