Don't be afraid to be blunt

When the world of Zendikar was visited for the first time there were beings there called Eldrazi. Ancient creatures that were powerful, hungry and devastating, the enemies of the Zendikar's plot. The cards that were going to represent them had to comply with three properties: feel alien; be enormous; feel hungry and voracious. A rule that Mark commonly follows is that whenever a mechanic or idea is to be put in a set, it has to be part of the common cards. In Magic the cards are made at different rarity levels, being common, uncommon, rare and mythic. The quantity of each card depends on which rarity it is printed at and this has both commercial reasons as well as power level reasons, with some effects being restricted to rare cards. The reason to put a higher priority on common cards is that the form the bulk of the cards of the market, therefore common cards are the main vehicle to deliver ideas and mechanics.

This creature is a common card, gigantic, with high power and toughness. They usually don't print common creatures with 8/8 stats. This one is an exception because it doesn't have very strong abilities that would be too strong for a common. Also, the mana cost is 8, which isn't cheap in magic. The card's frame is transparent, something that has never been done before this set. The art depicts an alien looking creature. The creature is colorless but it's not an artifact, something that has never been done before too. The mechanic "annihilator" is explained in the card's reminder text. In conclusion, the card is successful in feeling alien, monstrous and hungry.

A small issue showed up during play testing. The less experienced players were not attacking with the creature. Why not? Wouldn't they be compelled to attack with the creature having annihilator? The lack of experience explains it. Players with low experience would play this creature late in the game, when the board is more likely to have more creatures and therefore, players were caught in a difficult decision to be made. They were afraid that by attacking they would make a mistake, lose the creature in combat and throw themselves in a dangerous situation. The creature has a high cost and it's natural for inexperienced players to feel afraid of losing the high investment they have just made. It's curious that the creature itself is meant to scare the opponent, but it ends up scaring its owner because they are afraid of losing it or losing the game.

The solution was to give it another ability that forces it to attack whenever it's possible (the condition is there because there may be another card in play that prevents it from attacking). By forcing the players to attack with it the card is able to skip the decision that the player has to make and force them to see the value in its annihilator ability.

Mark likes to think about making cards as an art and artists like to be subtle. Artists learn to show, not to tell the whole story. However, sometimes subtlety doesn't work and players miss the obvious. The example comes from the set Mercadian Masques. In it some mechanics were tied to specific creature types. Certain creature types had a mechanic that was exclusive to that creature type. Even though the association between the creature type and the mechanic was tight, the players missed the fact that those creatures were introducing a new mechanic. Why? Magic players expect new mechanics to mean new keywords and vice-versa. When they broke that connection by skipping the keyword, the players couldn't notice that the mechanic was there, just not keyworded. This showed that, at least in magic, the relationship between mechanics and keywords is very strong.

Suppose that a level has an air plane and the player has to take it to get out. The areas behind the player could be on fire and yet, the player might miss the air plane. That's why games resort to arrows, an icon to highlight the air plane, a line "Run to the air plane!", or even all the previous at the same time. Even though something may be obvious, sometimes you have to take the player's hand to prevent the player from going into the wrong direction. That usually is the case when the game presents challenges that require jumping, avoiding traps or some other thing. Sometimes the player is forced to take damage to learn that that thing is a trap.

Is there a rule? It depends on the developer's experience and the audience's experience. It depends on the game's complexity. As a rule of thumb the players don't like to be forced to do something and lose their freedom of choice. Uncharted for example has chasing or escape sequences that the player has full control over the character, although he or she is still being forcibly directed to go in a certain direction.