I think Don Carlo(s) has some of Verdi's most powerful music, especially in acts 3 and 4. Philip, Eboli and Elisabetta have three of my very favorite Verdi arias; Philip's "Ella giammai m'amo" is a pinnacle of dramatic writing. His subsequent exchange with the grand inquisitor is unique. Only in a couple of places does Verdi fall into the old rum-ti-tum musical conventions that he finally left completely behind in his last two operas. Catchy as it is, and as poignantly as he refers to it later in the opera, the vow of friendship duet strikes me as one of those conventional moments. I'm not too keen on the auto-da-fe scene either.
It's quite a pessimistic opera, isn't it? Politics, religion, love, friendship - nothing goes well or ends well. Everyone is frustrated and miserable, and there's no light at the end of the tunnel. Only Carlo's and Rodrigo's vow of friendship offers relief from the shadows, and we see what becomes of them, although what becomes of Carlo depends on the version and the production. Schiller, I gather, had him dragged off to prison or worse. I suspect that Shakespeare, Verdi's idol, would have injected a little comic relief into the grimness of it all.
Today's performance wasn't one for the ages, but it put the work across pretty well. Nobody was awful except the so-called "celestial voice," which sounded more like a third-string Santuzza than an angel. I suspect the singer has been doing that role for about fifty years and they haven't the heart to retire her. I thought the conductor could have let the music breathe more in places. I guess that's a lost art. One of several.