But the autumn's most eagerly awaited exhibition, the one that is generating the biggest noise and by far the most frantic ticket sales, is the one that is, on the face of it, the most difficult: the dark and completely abstract paintings of a depressive, alcoholic and - let's be polite here - unphotogenic New Yorker who, even 20 years ago, would have been considered almost laughably abstruse - Mark Rothko.