Then there are genre-specific social contracts. Players of RPGs expect that their character will be more powerful at the end of the game than at the beginning. At the start of a game, a giant rat might prove to be a challenge. At the end, that rat should be less of a threat than a mosquito is to a mammoth. This is no doubt the reason that many people find the auto-scaling foes of Oblivion so underwhelming. In Oblivion, all enemies level up with you, meaning your relative power has not increased and, in doing so, the social contract of RPGs has been broken.
FPSs have a similar expectation in weapon advancement. At the dawn of the genre, players started with the weakest weapon in the game and went on to collect more powerful guns as the game progressed. Over time, it has become much more common in FPSs to start with an average weapon. This can feel more realistic, but comes at a cost, as looted weapons may turn out to be worse than the player's current one, breaking the social contract of advancement. Other advancement mechanics must replace it, such as RPG-like elements of character advancement. BioShock's plasmids and Sam Fisher's increasingly sophisticated equipment in the Splinter Cell games are two good examples.
Social contracts in gaming are not a one way agreement from designers to players. We gamers must uphold our end of the bargain as well. Gamers possess a slightly more generous suspension of disbelief than in many other media. We forgive the fact that the boss is wearing "plot armor." I saw my sniper round plug Zakhaev in the head in Call of Duty 4: Modern Warfare, but the plot called for him to lose an arm, so he did. And as for all those chest high walls that I can't jump over? Well, I won't mention them if you don't mention how many guns I'm carrying. Deal?
For game designers, it's important to recognize and adhere to the various social contracts in games, but doing so can be limiting. It requires design choices which, at worst, feel ridiculous like Gannondorf, or, at best, go unnoticed by the general populace. On the other hand, the best designs play with what gamers expect from the medium in novel, intelligent ways, such as the insta-deaths in Half-Life. Most consumers of games won't recognize that a social contract is even in place. They will only notice that something is wrong when their expectations are not met and then lambaste the designers for breaking the social contract.
Just like saying "Shit" in front of Mrs. Swensen garners the response: "Dude, not cool."