My husband's sense of humor is what made me fall in love with him. I LOVE to laugh and laugh hysterically...'til I am in tears and can't breathe. Life is too short to take it too seriously and I am too serious way too often; I require my other half to lighten me up! Even my close friends have an awesome sense of humor. I need that because I can be serious enough for all of us!
Taking life too seriously is detrimental to health and a good laugh or giggle just makes me feel better about life in general. So it's definitely somewhere in the top of the list of necessary/desirable qualities for a partner.
Well, it is important, but I find other things more important like whether or not they are respectful or caring about others and stuff. Those are most important and of course humor is, but that can be worked on a shitty personality can't.