Like looking for certain broad points of compatibility. For example, dominant and submissive traits are very fundamental to our nature, and too big of a mismatch here will be a source of conflict (two dominants) or unfulfillment (two submissives). There's a lot of wiggle room and nuance of course, but in broad strokes some people want to make the decisions, while others want to have those decisions made for them. It's important to take stock of where you want control, and where you want to give up control (and find someone who's somewhat close to your complement).
You'll want at least some common interests so that you actively enjoy some of the things you can do together and are not always doing things together for the others benefit (although that IS important some of the time). You'll want somewhat similar approaches to home life (a major source of conflict). Some compromise is healthy, but too much is stifling.
Honest, open and respectful communication is an absolute must in both you and your mate (with the odd slip-up that you make a point of correcting). Vulnerability basically comes with the territory here, since you can't be fully honest and open without becoming vulnerable. We all have our secret hurts and shames and desires and insecurities, and they'll always be a source of weakness if we can't trust our partners with them (after which they become a bond of strength).
Having a deep relationship with another human being is a humbling experience, because it's impossible without sharing empathy and burdens and trust. It forces you to live outside of your own little world, and I'm beginning to think that I likely wouldn't have been capable of such a thing in my youth, even if I'd known then what I know now...