Yes! to know the following
1. What their annoying habits are and if you can put up with it for the long haul
2. What it is actually like to live with that person, can you live with that person all day everyday and every night?
3. What they're like in bed (no I'm not just talking about sex, do they hog the blanket? Do they snore? Do they kick?
4. Division of labour? Who will do the household chores?
5. To find out if they are a slob or not, what are both your expectations on cleanliness?
6. Finances? Are you going to go halves on everything? Who will pay for the groceries and bills etc
7. For men: Do they put the toilet seat down after using it?
8. How much would you spend time together and how much apart now that you live together? Are you the kind of people who like to be joined at the hip or like to have a bit of space and quiet time?
9. Now that you live together how much are you going to have sex? Because for both individuals they may have different expectations.
10. If you have a fight, how are you going to deal and handle each other?
If you don't know any of the above before you get married and only decide to live together after you get married a lot of these things will come up whilst your adjusting to married life and also living together.