My ideal critique partner would have all the qualities Toni outlined. I, too, return my partner's work with equal priority.
My super-ideal critique partner would also join in brainstorming the sketch of a new plot. This 'ground-floor' familiarity would help her (or him) help me across those seemingly-impassible gaps between where I am and where I want to go as I am writing. Then, of course, he or she would still catch my lapses into omniscent POV.
I think it would be fun to write with a partner. (Can you tell I have Libra rising?) That means a compatibility of genre, vision, and style--not an easy match, but I believe someone is out there and will appear when the time is right.