I'll chime in!
We started dating almost 8 years ago. 3 years living together, 2 married.
The most important thing about our relationship is: We talk. A LOT. About everything. There is absolutely no taboo topic between us. Everything is subject to talk and think about and exchange ideas and views. Likes and dislikes, sex, crisis in the middle east, US economy, if X celebrity is or isn't hot, how you should or shouldn't cook. We dont have scheduled "we need to talk" about the goings of our relationship. We just talk freely.
One trait that support the above, we are both very rational. There is no drama involved.
We have fun together. We dont take ourselves so seriously. Our loving nickname for each other is something like "dumbass", if i wake one day with him saying "good morning, my love" i'll know something is wrong lol. At the same subject, we really rarely say "i love you", or things like that. We just do. It's so obvious on our day to day life that we dont feel the need to keep "reminding" ourselves.
We do chores together. Going to the supermarket, for example. None of us particularly like it, so we go together, make each other company and it goes easier. At the house we have some 'defined' roles, mostly by preference. I can't stand smells, so he takes care of cat litter box and cleaning expired food from pans and pots. His nose feels bad when dealing with dust, so I clean most of the dusty things.
We get angry over different things in life. May be funny, but this is important, because while one is raging, the other has a calm head to deal with the problem.
We absolutely dont try to change each other. We respect and talk, but there is no nagging/bitching about things. For example, he smokes. It is not pleasurable to me as a non-smoker, but I'll never "make him stop". Obviously i would prefer if he stopped, but it's his choice. He will make it the less annoying as possible to me (smoking outdoors, puting a fan to take away the smoke...) and that's it.
Companionship. There isn't a problem to be around each other, we dont get 'sick' of the company of the other. We like and enjoy our time together. I don't understand "man caves". Our cave is at the living room, together. If he want to play videogames, I will play at the computer, or read a book, and vice versa. But we are there, together, talking some times, or just being there.
A dealbreaker would be drama. Gosh, i hate drama and having to walk in eggshells with someone. Lack of honesty, also. If you can't be open and honest with your partner, you have no reason to keep calling him "partner", imho.