Having someone who can do what you can't and who can't do what you can. If I married someone who was basically a female version of me, I wouldn't need her because we would both be good at the same things and bad at the same things. From what I have seen, the best marriages are between a man and woman who each need what the other provides... and who both realize that.
Quote from an Arlo & Janis comic - they were sitting out on the patio in the afternoon, and Arlo remarks about how beautiful the sunset is shining through the branches of a tree. Janis replies that the tree limb is rotten and needs to be cut down before it falls down. Arlo thinks to himself "Some people would say incompatible. I prefer complementary." There's a lot of wisdom in some of those Arlo & Janis comics...