I'm not going to vote in the poll, because as usual they are too limited in scope for the topic.
Monogamy is important, as the first thing. It is a waste to spend any amount of time with someone who is not on the same level as you. If they are not serious, you are wasting your time and sooner or later it is going to hurt. If you have children, between the two of you, how important is it to you to know that your heir is really just that. The product of the pair of you and not someone elses. That is truely theft of the heart and the wallet when it occurs.
Level headedness is a quality you don't appreciate till it is not there. Not just lack of flightyness but stability of emotions. I won't spend my time on this earth with someone who only knows how to raise hell and enjoys it.
Fun? There is nothing wrong with fun; it adds spice to our daily lives. Only, our daily lives are made up of the every day ordinary stuff. I crack a joke and enjoy a good joke or humour where it is found. (Though that isn't all that makes up fun) The reality is when you wake up and look over at your pardner in life, who is just waking up also and realise this is a beautiful lady. (Before the daily morning rituals) When it might be real nice to go make coffee and bring a cup before waking him/her up as a nice suprise. Or waking through the yard, you think of that one who is special and pick a flower to bring in unannounced just because. That is reality, that is the everyday that each of us live within. That is what you have to deal with on a daily basis. Fun comes and goes. Reality is what stares you in the face everyday.
Truthfulness? Goes hand in hand with it. How can you base a relationship without it?