Me and DP have never married, neither of us interested in the idea, but 20 years in, and still very happy.
To answer you're individual points:-
1 - Sheer dumb luck. I was only 23 when we got together, and had ruined most previous relationships with my immaturity. Either got bored and moved on, or didn't put enough effort into the relationship and so the other person dumped me.
Met DP in a club, went on a few dates and we hit it off. 10 months in I'm just starting to feel the usual itch and suddenly there's a baby. DP had had a phantom pregnancy, first she'd known about it was going into labour. First I knew I was getting a phone call from her sister that I had a daughter.
I decide I need to grow up, 6 months later we're living together as a family. Luckily, me and DP fit together pretty much perfectly. I can't imagine the situation having worked out had I been with any of my previous partners.
2 - Feeling supported emotionally is great, we've always got each others backs. As a previous poster said, it's never me vs. DP. Always me and DP vs. the problem.
Intellectually, isn't something we really do. We have very very different interests, DP would rather pull her own teeth out rather than talk about politics, and I've got equally little interest in anything the Bronte sisters have written. That's fine, it's what friends are for, and we never struggle to find other things to talk about
3 - Some times it's great, sometimes it's bad, sometimes it's non-existent. Right now, 20 years in we're in a "great" phase
4 - We're both very direct communicators when it comes to problems in the relationship. It comes from having that one very huge problem 10 months in. We sat down a few weeks after DD was born, and had a long conversation about logistics, what her moving in with me would look like, what she needed from me to feel secure enough in doing so, what my parenting looked like while she was still living with her parents etc.
We do the same now, if there's a problem we sit down, identify the issue, identify possible solutions, then work out which solution is best and who's going to implement it. If we can't come to an agreement then we go away for awhile, come back to it later and one of us compromises.
We do not shout, we do not call each other names. DP has heard me raise my voice exactly twice, and both times it was to get DD's attention to stop her doing something dangerous. If either of us get frustrated with the other, we discuss it. Only once has it ever bubbled over into anger, and it was because I steamrollered over DP's opinion and did something without thinking of her. We learnt from that and haven't repeated it.
You've asked other posters about their parents relationships @Christl78 , so I'll answer that as well. DPs parents had a good relationship on the surface, but hard choices tended to get swept under the carpet and not dealt with until it was too late. They loved each other hugely though, until the day DMIL died.
My parents relationship on the other hand was awful. My Dad is a twat, and shagged his way round half of the UK, (including my DMs sister). My Mum should have divorced him way before she did, but stayed together "for the kids". Neither me or my brother appreciated that, it's a hell of a cross to bear to know that your mother spent a third of her life miserable "for you"