The art of the opera putdown
"...imagine René Kollo on his USUAL form as opposed to his freakishly reasonable performance in Dresden." - Davyd Melnyk.
There is a subtle art to the opera putdown, and I think that a brief primer is in order. If you can apply all of these approaches, you will be quite the opera "expert," or at least fun in the nosebleed seats. All of these are presented with an ironic smirk, but - even the most offended party will have to admit that they work. Well.
1. The "good, but..."
"Sinopoli's Ariadne auf Naxos is quite good, but Sinopoli was never the Straussian like Von Karajan or Böhm."
This works by admitting that something is technically good, but lacks some intangible that would make it really "good." This implies some broad, and great, knowledge of a lot of artists. Solid for boosting your cocktail-party expertise on the matter.
2. The Golden Age
"Siegfried Jerusalem is a decent tenor, but lacks the real Heldentenor approach - like everyone since Melchior."
Things were so much better when singers you've never heard either live or in good sound were active. Oh, snap! Reserve this one for curmudgeonly pronouncements about the state of things. It is a doom-and-gloom approach if ever there was one.
3. The "Poor Dear"
"Deborah Voigt, the poor dear, has never been right since they cut her stomach up."
This implies both a real basis for your criticism and a basis solidly beyond the control of the singer. Somehow, I prefer this one, as it makes me sound authoritative within the context of both knowing the artist and knowing them before they had their mishap.