If you asked me to imagine a kind of quintessential American opera-going experience of, say, 50 or 100 years ago, I might envision something not unlike Washington National Opera’s new production of Camille Saint-Saëns’s “Samson and Delilah,” which opened on Sunday afternoon at the Kennedy Center. The show wasn’t completely retro; both scenery and staging had a modern, high-tech sheen. But the scaffolding was traditional: big singing, broad acting, basic staging, taking the art form as a vessel for vocal glory and musical glamour. On that wavelength, there was a good time to be had.