In the past week, chaperoning a young supernumary, I've heard a rehearsals and two performances of Don Carlo, which has been ample reminder that, in the 19th century, opera was the principal genre for innovation in orchestration and that, among opera composers, Verdi was an orchestrator with only one major competitor, Berlioz. (Yeah, Berlioz, I've been wanting to add Les Troyens to my list of landmarks for long, long time but have been more than a little frightened by the prospect of writing something, even something very brief, about the grandest of the grand operas; the piece, as a whole, and in so many details, is more than remarkable.)
In any case: Verdi, orchestration. Pay attention to these things: the economy of his ensemble writing, allowing for the most potent use of the entire range of scoring patterns, from solo instrument (in Don Carlo, exquisite writing for clarinet and cello in particular) to chamber-scale groupings all the way up to full orchestra, choir and soloists, a combination that registers more on the memory than actually appears in a score, then his usage of the on-stage bands to achieve very subtle spatial distinctions, and his reserve with the use of the full string section, with the weight of winds, particularly solo winds, but also the horn and trombone (agile, originally for valved instruments) sections, used as sharp points of contrast. To some extent, these are features of orchestration that we have more or less internalized as a standard of good practice, but the degree to which Verdi is responsible for that practice is either overlooked or taken for granted.
The 20th century started out blockbusters for opera orchestration, between Debussy and the two Strauss Einackters. Why then, with the exceptions of radically different pit bands for Glass or Ashley, so late in the century, did opera so lamentably stop being a genre for instrumental innovation? I suspect that the "too big to fail" scale and status of opera houses and orchestras as institutions played a major role, with considerable inertia in the composition and usage of the pit orchestra. In any case, the failure to innovate has come at a large cost to the genre as both a wider public attraction and a musical/intellectual playground.