It's not available online, but Justin Davidson (no relation) has an extremely interesting essay on the modern art of conducting in this week's New Yorker. (My only disappointment with it was that he didn't mention the argument Alison Winter makes in Mesmerized: Powers of Mind in Victorian Britain about the ways that orchestral conducting develops in parallel to other kinds of action-at-a-distance stuff, everything from mesmerism to hypnosis and anesthesia--think Trilby.)
Anyway, here's a sample of Davidson, who's writing here about the effects of things as various as the expansion of the musical canon, the rise of musicians' unions and the maestro-nomadicism enabled by the jet plane:
Precise gestures produce precise sound, and the blossoming of technique after the Second World War was accompanied by a musical fetish for clarity. Pierre Boulez developed a fearsome gestural repertoire for executing the hyper-complex rhythms of modern compositions, including the ability to mark a different beat pattern with each hand. Boulez's approach, which arose as a response to the specific demands of modern compositions, soon spread to music of all periods, because it matched the needs of the global music industry--the high-fidelity LP, for example, captured every flaw in an orchestral performance.
The musical consequences of this trend have not been entirely salutary. Furtwangler relied on degrees of vagueness, especially in Strauss, where he regularly allowed the edges of a chord to bleed, and let the waves of fast fiddle notes gurgle indistinctly. Nowadays a hazy softness is judiciously applied in performance--Rattle does so exquisitely in Debussy--but it has become a special effect. Rattle's performance of "Ein Heldenleben" with the Berlin Philharmonic at Carnegie Hall in January was full of precision-magnified detail. You could make out the highlights on all those crystalline tremolos and follow the curve of each dewdrop pizzicato. Modern conducting sometimes feels like a glass skyscraper: initially thrilling, but finally irritating, in its relentless sheen.