What qualities should a significant other have?

Those that will best suit what you have - both complementarily and contrarily. They should be similar, but different; your equal - but not too equal. Some would say that the significant other should be that person who gives you the following realisation: "I am/was complete. I never knew there was anything missing until I met you." In other words, the half that you never knew was missing. That might be too much responsibility for one soul, and perhaps too romanticised, but still, it's one way to look at it.