Make sure you know this stuff to make sure it goes smooth.
1. Their political affiliation.
Regardless, you should probably avoid the topic, but it's good to know whether you're in the same basic camp or not.
2. Any taboo topics.
AKA don't ever mention ex-husband/alcoholism/'Nam.
3. Their relationship with your significant other's ex.
Did they hate them? Were they neutral? Or will they spend the entirety of your relationship missing the last significant other, and "accidentally" calling you by their name?
4. What they do/don't know about your relationship.
"Ever since we moved in together" "You.....WHAT."
5. How you should address them.
Mr. Smith? Dr. Smith? Gary?
6. Their occupations.
Easy conversation fodder, so you can ask them about work. Or not ask them about work, if, they don't.
7. Are they a sports family? A music family? A travel family?
What's the common ground going to be here?
8. Are they "cool" parents?
In other words...how down are they to party. (Disproving of you drinking? Or disproving of you not partaking and, ahem, acting "stuck up.")
9. How do they feel about PDA?
Uncomfortable? Or worried if they don't notice enough of it?
10. Are they religious?
Will they say grace before dinner? Do they keep kosher? Do they avoid alcohol? Make sure to get a heads up!
11. Are they okay with sharing a bedroom?
Some parents are, some parents aren't, and no amount of turning 18-30 will change that.