Marriage is a big decision that people don't know enough to make when most people get married. From my experience it take a mutual direction in life to make it work. I think both people should be in agreement about what they want out of life and what they want out of each other.

1. Do each want children.
2. Money and careers. (do both work, professionals, blue collar, slackers?)
3. religion (people of different religions can get married, but will this be an issue for them?)

Those are just the broad strokes life decisions. To complicate things, people change their minds over time as well as personalities. I think at a bare minimum to make things work you have to be able to communicate well. You have to have a mutual trust and respect, you have to be able to let go of anger and contempt for your spouse. I have an aunt and uncle who hold so much anger and resentment for their partners they can't resolve anything. It is like 2 pit bulls in a cage. They constantly seperate and get back together because they miss the company and good times they had.