If they would still love you if you lost a leg. Or both legs.
Seriously, the following things need to be discussed (either formally or informally) prior to considering marriage, I think:
Money/spending habits, and if there are any compromises to be made in this area.
How much interaction you'll have with your in-laws (there should be no mystery as to what you're getting into with this one).
Religion/spirituality, and the importance it will have in your lives.
If children will be in the picture, and how many.
Importance of career. Don't expect someone to drop their career pursuits just because you get married.
If you can tolerate each others' pets.
Division of labor in terms of child rearing and household chores.
Geographical location, if one of you has to move.
Sex. Enough said.
Amount of personal space/room to pursue individual interests each of you requires.