Well, let's take Scream.
It was something new at the time... while snarky/self-aware dialogue, pop culture references, and so on are commonplace now, that really all started with the Scream movies. Then Buffy the Vampire Slayer came along, all the lower-budget genre shows and movies started (trying) to imitate Joss Whedon and Kevin Williamson's dialogue, and the rest was history.
The point, though, is that -- at least initially -- Scream brought something new to the table. Add to that the fact that it was made by a respected horror film director, that the quality of the acting was probably above-par for that "type" of movie, and that it actually gives its female lead something resembling a dramatic emotional arc... well, there you go.
OG Nightmare: Again, it was something new. People probably didn't even really pigeonhole it as a "slasher" film initially -- it was the endless sequels, Freddy-themed merchandise (toys, video games, etc.) and, eventually, persistent rumors of a Freddy vs. Jason movie somewhere on the horizon, and now it's classified as a slasher franchise. But if it had just been that first movie, I doubt that would have happened.
Black Christmas: Takes a unique approach to the characterization of its killer, gives the heroine a compelling (and politically progressive) emotional struggle to deal with, isn't overly or graphically violent, and has some humor in it that's a significant cut above the "Huh-huh, Jimbo can't get LAID" jokes we get in most of the Friday the 13th movies.
Halloween: Halloween wasn't the first slasher film -- Hell, that genre didn't even really exist yet -- but it was telling a story that was still fairly fresh and unique. Laurie got a little more characterization than a lot of later Final Girls, Annie and Lynda were at least fun, Donald Pleasance chewed the scenery, spat it out, then fed it to his offscreen Ferengi bride... mostly, I think, it's the fact that John Carpenter and Debra Hill had to create new characters; they didn't have a bunch of stock archetypes to endlessly clone like the later movies did (the shy/bookish girl, the nerdy guy, the jock, the outcast, the practical joker, the slut/tease, the stern authority figure, the randy couple that keeps sneaking off for some hanky-panky; you get the idea).
And that, I think, cuts to at least part of why the slasher genre has taken so much heat: because it's seen as imitative, as "ripping off" (for the most part) earlier, better films.