I read the usual Restoration plays in an edition that introduced them as clever but heartless precursors of Sheridan and Goldsmith, and now I've read Sheridan's plays in an edition that insinuates that they are bowdlerized and insipid descendants of Restoration drama. (Why don't people edit books they like?) I'm more sympathetic to the latter point of view, at least re Sheridan; Goldsmith is a different kettle of fish altogether, and not Restoration-like at all. (Sheridan, it appears, actually bowdlerized Vanbrugh's Relapse to make it acceptable to a 1760s audience.) Both The Rivals and The School for Scandal are comedies of manners; the plots -- e.g., man woos idiosyncratic woman with comic aunt -- are very much in the mold of Congreve and Wycherley. The plots are clever enough, but both plays are short on spirit and edge; the characters are pretty blah, though I imagine a good actor could make them come to life, and the dialogue, despite stunts like Mrs. Malaprop's malapropisms, never gets off the ground. In general these plays are what one would expect of period pieces -- unlike The Country Wife, which is entirely contemporary.
The difference, it seems, lies in the smut. Are plays without explicit sexual references ipso facto quaint? There's something appealingly reductive about this notion, and it certainly is quite hard to find sexually explicit ancient texts that are boring. I'd venture an alternative explanation, though, which is that the basic dramatic tension in the comedy of manners -- the horror of being cast out of the loop -- depends on social existence being a bit of a tightrope-walk, which requires a degree of cruelty and willingness to ostracize on the part of the "men of sense" (as ostracism would otherwise be empty) as well as a degree of objective danger (as the men of sense must appear sensible). Sheridan's tendencies deprecated the former; the age's conventions prevented the latter, and one is left, in the end, with two halfhearted plays that were unable -- unlike She Stoops to Conquer -- to create a genre to match their temperament.
The School for Scandal is interesting and flawed in the way that Auden found Twelfth Night interesting and flawed: a few of the characters -- in this case, Sir Peter Teazle; in the other, Viola -- exist at the wrong level of seriousness, and intrude rather damagingly into the fabric of the play. Sir Peter is an old man who has married a young woman who is about to start an affair with his hypocritical ward who's "gulled" Sir Peter; in a Restoration play this would have been a purely comic part, but Sheridan lacks the heartlessness to make it work, and there is an entirely jarring degree of pathos to the scenes in which Sir Peter appears -- jarring because he is so much more "real" than the others; because his presence critiques and undermines the scandalmongers; because it is clear that in Sheridan's view the entire "school for scandal" is at some level out-of-date and irrelevant, like the women in Pope:
As hags hold Sabbaths, less for joy than spite,
So these their merry, miserable night.
Still round and round the ghosts of beauty glide,
And haunt the places where their honor died.
And because these facts let the air out of the main action of the play: if sensible people are indifferent to scandal, the activities of scandalmongers can only be so important; but if so almost everything that happens in the play is insignificant.
I should say that by contrast The Critic is an entirely admirable and very funny play, much better than its Restoration model The Rehearsal.