Any positive, forward-going opinion on how to improve a given area, can also be viewed as what you describe as being a complainer.
As far as I can tell, people tell between complainer and positive thinker in the following algorithm:
Is it a manager? --> positive thinker
If not a manager- Is the person at least remotely attractive? --> positive thinker
Anyone else --> complainer
I remember being truly shocked when co-worker, and a friend, told me that I was a 'complainer.' My argument to him was that I was correct in my complaints and that my criticism was accurate, justified, and deserved. He said that I usually WAS right but that didn't mean I wasn't a complainer. And, quite honestly, I still have trouble understanding that.