Star has been taking a beating over the past couple of years. With a BK exit and the soon-to-be announced departures of US/JJ(/and likely TP) for OW. Despite having more airlines in their alliance, I cannot think of a single reason why Star would still be better than OW. I guess, maybe if you are going to China or Mongolia. That aside, there are some major holes in the Star route map. IMHO, OW acquisitions now represent quality over quantity. This seems to be lacking at the other two alliances, which will take just about anyone.
You don't need to go to China or Mongolia -- just Europe. Even leaving US out of the picture, you've got a lot more destinations on UA than on AA. As a big network carrier, LH is a lot easier to deal with than BA; as major secondary options go, the combination of LX, OS, and AC provide a lot more flexibility than just IB (and I guess . The result is a lot more chances of finding a good deal, a good schedule, or an available premium award seat.
OW is the only alliance without a Korean carrier. It's the only alliance whose US carrier doesn't maintain an Asian hub. Those are all competitive disadvantages. That's not to say there aren't advantages -- South America, obviously, being one -- but if given a choice between an alliance strong in Europe and Asia, and another strong in South America and Oceania, more people are going to choose the former.
As for the "quality over quantity" argument: AirBerlin.