Technically, Ning is essentially a listing and linking plus skinning platform, simple yet flexible enough to meet quite a number of simple application needs. Functionality of Ning-based applications are not feature-based but people-based just like real world town markets.
I think Ning is a good thing because it enables users to create and experiment with services they need or ideas they had quickly and cheaply. As to their business plan, I have some doubts but I can also see several paths they can follow to reach the green acres. Execution wise, I think they need to improve on it greatly. Scalability? No big deal IMHO if the capital and the talents are there. Platform business? Not really. I think Ning is more of a crossroad town that could turn into a boom town.