By spiritual I mean what you believe about God, which festivals you like to celebrate, your opinion of tradition, whats acceptable in the name of rites and what is the definition of superstitious/orthodox. When there is a difference in these, then the compromise is on something that is deeply ingrained.
By physical I do not just mean sexual, that is a prerequisite for a healthy relationship. There has to be a chemistry, spark that draws you to your partner like a magnet. There must be something in the looks, moves, laughs, or smiles; something that compels the need to be with him or her.
By social I mean family patterns, social life, finance objectives, domestic responsibilities, communication style, flexibility to oogle at opposite sex:), intellectual inclination and opinion towards rearing children.
Basically what I am trying to convey is similarities make life together much simpler. Living together involves compromise, and it is easier to compromises when you share common values and interests. Sure, sometimes opposites can attract, but for a stable relationship I would bet on similarity.