LIKE ANY MAJOR METROPOLIS, Los Angeles has its normal sea of troubles, but there are two fundamental problems that really define the city and the challenges it confronts. The first, with which the Weekly has dealt extensively of late, is the quality of its air. The second, which may be even harder to fix, is the quality of its economy. Over the past quarter century, Los Angeles has been downwardly mobile, with its middle class shrinking to a fraction of its former size. Both these problems - air quality and, even more, the vanishing middle - afflict the nation generally. But Los Angeles has opened such a wide lead on every other city that we're not just quantitatively different; we're qualitatively in a class by ourselves.