I think the number one thing to look for in a partner is that you can communicate with them. I know that sounds goofy to some, but if you can't talk with the person about anything and everything, you might want to think twice before committing. Notice I didn't say that you have to agree with the other person all the time, but you do need to talk about a wide range of subjects.
Another important thing is that you can see yourself growing with the person. We should all grow as people. That's super important. But when you are with another person, you can either grow together or grow apart. Obviously you need to find someone that you can grow together with to make it for the long haul.
Another key is in how the other person sees you. My wife sees me as I am. She doesn't see me as she wants me to be. She doesn't see me as she wishes I were. She sees me for me. She sees the extra pounds, the dark places that I sometimes go to in my head, the stubbornness that I'm sure irritates her sometimes. She sees all of these things and more, and she stills loves me. That is such a key part of why our relationship works. And when I look at her, I see a better version of myself reflected back at me. She makes me want to be a better person. And that is definitely something to look for in a spouse.