Money
This is probably the biggest issue to sort out before making the leap into co-habitation. Honestly, there is no single correct formula for deciding who pays for what. But you do absolutely need to be comfortable discussing it openly and candidly. Don’t avoid talking details. In fact, don’t neglect to talk cold, hard logistics throughout the process of merging residences (or even while just entertaining the idea); sure, moving in together is romantic and emotional, but it’s also about the coming together of two lives and there are a lot of boring details that need to be hammered out. And skipping these talks will do nothing but result in confusion, stress, and failed expectations down the road – nothing less romantic than that.
Where you will live
People often think this is a simple thing to figure out, but it’s more complex than it appears. Even if one of you obviously has a nicer/bigger/more advantageously located residence, that doesn’t necessarily mean it’s the right place for the two of you to begin your domestic experience. Moving in together means sharing a space, and no matter how much time you’re spent at each other’s homes, or how many sleepovers, or even if one of you barely goes back to your own place at this point, the minute you officially move in, you are living in their space. To some degree, you will be aware of this. And there are power dynamics that come with it.
There will be moments when the person who lived there first will feel unavoidably territorial (and then probably feel guilty and confused about feeling that way), and the person who moved in will feel less at home, or less of a sense of ownership over the space. Ideally, moving into a brand new place is the way to go; neither of you has any history nor subconscious sense of seniority there, and the more you can do to establish balance and equality, the better. If you do decide that one of you will move into the other’s place, make sure to talk about these potential issues beforehand, and commit to revisiting the issue anytime one of you is feeling a little off about it.
Alone time
All couples go through that phase where they literally feel like they can’t get enough of each other. And then they settle into a period where, although some of the voracious compulsion to eat each other’s faces has settled, they still really like having each other present pretty much all the time. Eventually, you make it to a point where you start wanting your alone time again. It doesn’t mean that you love the person any less, nor does it indicate any dissatisfaction with the relationship – it just means you’re still you, and you still occasionally need your space to do you.
It’s a good idea to let your relationship get to this point before putting both of your names on a lease. Sometimes the reintroduction of personal space and alone time can create friction within a couple; one person maybe comes to feel smothered, or neglected, or the communication is off in terms of expressing when you need to be alone, and for what reasons. It’s an adjustment period that every couple goes through at some point, and most make it out the other side even happier and more connected and more understanding of one another, not to mention much more fulfilled in a sustainable way. There’s just a lot less pressure when you sort out how to give each other the right amount of space when you aren’t already living together.
Pooping
I am of the belief that one of the key turning points in a relationship is the moment you let the other person be aware of the fact that you do, believe it or not, poop. I’ve known people who have gone entire relationships with their partners having no tangible evidence that they poop. Like, obviously they know it happens at some point, but they’ve never been able to identify a single moment where they know “yup, he/she is dropping a deuce right now.” Point: Before you live together, you damn well better be sure you’ve already gotten past the point of poop comfort.
Expectations regarding marriage, etc.
Some people have no interest in ever legally marrying their partners, so for them, making the move to cohabitation might represent the highest degree of combining lives. For other people, it’s a sort of “trial run” before deciding to get married. Some people see moving together as the step that comes before getting engaged. Some couples might just be moving in together temporarily. The bottom line: there is no one set of rules for every couple. There are a million ways to view living together, just like there are a million options as to how to love someone, or how to structure a relationship and build a future. You get to create your own life. The only key to making it work is to be very clear about what you want, how you perceive the steps you’re taking together, and making every effort to keep you and your partner on the same page. Before you move in together, make sure it means the same thing to both of you, and that you’re both in a position to get what you want out of it. 
