Two comedies of manners in two nights: one (Full Circle), written in 1952, makes an impact only because of its production values, whereas the other (The Rivals), written in 1775 and having similarly high production values, still has an impact in its own right.
It's partly the language, of course. Sheridan was writing at a time when the English language still had the freshness, complexity and, indeed, excitement of Shakespeare's time, when characters could be delineated by their use of language (compare, for example, Lydia Languish with Bob Acres), and when the drive towards simplification had not impoverished everyday speech. Sheridan's language, of course, lacks the depth and density of Shakespeare's (or Marlowe's, or Ben Jonson's), but it still has a subtlety and wit which makes it a pleasure to listen to today.
But it's more than that. The subject matter remains relevant today, unlike that of Full Circle, which seems so incredibly dated just fifty years on. Teenage girls still get silly romantic notions, just like Lydia Languish - different ones, of course, but still silly and still romantic - and they can still be equally stubborn and rebellious. There are still those like Falkland who are pessimistic to the core, questioning their own good fortune at every turn and killing their own chances of happiness in the process. And the ignorantly pretentious, like Mrs Malaprop, are still with us, as are those, like Sir Antony Absolute, who seek to bully their way through life, determined to get their own way.
Effective and lasting comedy of manners is based on people rather than the ephemeral mores of a particular time. That's why The Rivals has lasted so long - and will continue to do so, unless people change considerably.