Moving in together is usually decided on feeling and then lived out in logistics. An hour of unglamorous conversation beforehand prevents most of what goes wrong.
Money, specifically
Not "we will split things" but who pays what, what happens if one earns much more, and what counts as a purchase you discuss first.
Mess, honestly
Tidiness gaps cause more daily friction than anything else on this list. Both people should say their real standard, not their aspirational one.
Guests, sleep and alone time
How often people stay, what time the house goes quiet, and how much solitude each of you needs. All boring, all responsible for real arguments.
What happens if it does not work
Deciding in advance how you would unwind it is not pessimism. It is the thing that lets you be honest later instead of trapped.