Today was the 114th anniversary of the birth of Kirsten Flagstad, one of the biggest stars of the '30s Golden Age of Wagnerian opera singing. It's hard to believe that there was a time when Wagner's popularity rivaled or even surpassed Verdi or Puccini, but in the '30s at the Met, there was a huge amount of Wagner every season. In part this was a case where supply drove demand: there were an unusual number of singers who could handle Wagner's music and had the stamina to get through these operas -- not many, but more than there were in Wagner's own lifetime. Combine that with the feeling that Wagner's operas were uplifting and noble (they do have all that redemption jazz going on) and decades of proselytizing for Wagner by eminent critics, and you had a period where Wagner's operas were true repertory pieces -- as opposed to today, when many of them require a certain amount of special pleading. (Companies want to put them on, and mounting a Ring cycle is proof that you're a major-league company, but most of his operas simply aren't big box-office like Puccini, some of Verdi, and even Mozart, whose operas were only just starting to come back into fashion in the '30s.)
This era of Wagnerism in America ended, of course, in 1941; by the time the war was over, Wagnerian singers in any case weren't as plentiful as they had been.
The way Wagner's operas were staged in the '30s at the Met and other big houses helped create the cliches that are familiar to people who have never seen a Wagner opera: though some European houses had already tried to take a more modern or symbolic approach to Wagner (like Mahler's production of Tristan at the Vienna State Opera), the Met apparently went in for spears, helmets, literal sets and broad acting. This clip of Flagstad performing in The Big Broadcast movie seems like a good representation of a time when these cliches were still taken seriously by opera fans. (This is not a knock on Flagstad, a great singer and, by many accounts, a serious and involving performer.)
I'm not, myself, a big Wagner fan, but I like several of his operas very much, especially Tannhauser and the opera that's supposed to be the most inaccessible of all, Parsifal (it's long, weird and somewhat puerile, but the music is incredibly hypnotic and the characters are quite fascinating). I think my least-favorite Wagner is actually his crowning achievement, the Ring, which I think has some serious flaws as theatre, as philosophy and as most importantly as an expression of character in music. Opera is fundamentally about bringing out character in song, but Wagner's method of "endless melody" -- meaning, basically, recitative over orchestral development of important motifs -- often seems to fall into set, even predictable patterns, so that different characters sound very much the same when they sing. Richard Strauss used the Wagnerian method but put a lot more individuality into the vocal lines; Wagner sometimes sounds like he's uninterested in the most important instrument of all (he writes very demanding music for voices, but not necessarily distinctive music). This is not the case in every scene in the Ring, but it does happen in some parts.