I like the observations provided in the posts, and wish to contribute my own:
Is the person basically a Good person? By that I mean respectful, honest, with integrity, kind. Shares well, looks out for others.
Can you communicate? And can you put the relationship in front of being right? As I say to my 5 year old son - it’s not whether you make mistakes - because you WILL - it is how your recover - and recovery is about honest, open communication and a willingness to commit to change and compromise…
Beyond that, anything goes. Having a best friend in your spouse is incredibly cool, but not if the person is not a good person, or can’t communicate effectively…over the long term, you can learn to be better friends, but integrity and communication are harder to learn…
A few more things to discuss, but are generally not dealbreakers:
What temperature can you and your SO both live comfortably with? I’m really tired of turning the thermostat up and down and ceiling fans on and off trying to find a happy medium.
Pets. How many, what species, indoor or outdoor?
Willingness to compromise (okay, this might be a breaker). You can be the most easy going person in the world and able to tolerate most anything, but once in a while you’ll be adamant about something that you want just so. Make sure you and your SO can work on these types of disagreements.