I have f.f. points in AA but it seems difficult to use them for overseas flights some times. Star Alliance seems to be growing and partnered with so many airline options. When going to Europe I find that the partners I can only work with AA and British Air. Why has One World lost so many of it's previous partners??
It's only lost 2 so far, Canadian Airlines in 2000 when it was taken over by Air Canada, and Aer Lingus last year because of it's shift to low cost carrier status. Star Alliance has lost three, Ansett, Varig and Mexicana (which is about to join OW), SkyTeam's about to lose Continental.
Oneworld doesn't have as many options in Europe as the other two, but what doesn't work for you works better for others. It has better coverage than the other two in South America and Australia for starters.