The opinion of a lifelong horror fanatic, here...not necessarily of slasher flicks, but I've seen more than my share and here's what I would say would make for a successful one:
Slasher movies are about four things; chicks, body count, grue and the killer.
Chicks - lots of 'em, the more nudity the better.
Body Count - the higher, the better. No time for characterization here, no sir! Parade lots of morons across the screen (especially obnoxious ones) because you can't wait to see them get what's coming to them and the slasher fan will clap and chirp like a monkey playing happily with a wad of his own poo when the obnoxious morons are obliterated onscreen.
Grue - the more creative the manner of death, the better. Slasher fans get bored when their slasher heroes stick to just a machete, or just an axe...we're talking weed-eaters, hacksaws, bandsaws, chainsaws, lawn mowers, cork screws, head-crushing, decapitations, disembowelments, and everything in between. Also, if you want your slasher flick to be successful...DO NOT just show "aftermath." If the slasher fan doesn't see the murder happen onscreen, plain as day, no last-moment cut-aways and no "cleverly hidden in the shadows" mayhem, then you'll have yourself a successful slasher flick.
The Killer - has to be big. Has to be intimidating. Has to be an unstoppable juggernaut with more lives than a cat. Must be masked. However, must not appear oafish or clumsy during his kills. Must prove handy with a variety of murder weapons (see: grue), and during the big finale, he can't just fold like a wet noodle when the heroine puts up a fight. He's "the killer" for a reason, y'know.
The only slasher flick which proves to be an exception to most of these rules - and which immediately comes to my mind - is the original Halloween, which is pretty much minimalistic horror at its nigh-finest.
*Note: none of my comments above are intended to offend fans of slasher flicks...though for my money, there's a bit more to horror than a hockey mask or a razor glove.
