Many years ago in my software engineering course, my professor said that he would add to Fred Brooks' concept of a "surgical team" and said that he found that many successful engineering teams had a "team mother": the person that helped quell disagreements, remembered birthdays, cheered the successes no matter how small, kept tabs on how folks were doing emotionally, etc. Years later in my professional career, I found this to be true and met a few people like this on my teams -- not the best engineers, but additive to a great team.

I think that successful teams may have a place for a "team canary" (?): someone that is going to speak up about points of friction where most others might just end up with apathy and learn to deal with the inefficiency or friction (a "that's just how it is" attitude). Sometimes, complaints are a sign of genuine friction. Complainer may not feel like they have the authority/allotted bandwidth to remove the point of friction. When this happens, give this person some ownership of the friction points and see what happens.