That's always been my favourite horror film. If you could pidgeonhole it as such. It's one of a kind (no matter how many times they tried to reproduce it in the sequels). What I really liked was how, in terms of what was on screen, it very un-gruesome. Everything is implied and your mind builds on it. It's a psychological horror but it's so in your face at the same time that the description quite doesn't fit. I think it's an absolute work of genius. I remember watching it one time when I was very sleepy and couldn't keep my eyes open. If you watch it with your eyes shut, it takes on a whole new form of disturbing. All you can hear is a chainsaw and a chick screaming for most of it. And yeah, it was at the same time a very black comedy, from the whiney crippled guy (I couldn't help but burst out laughing when he falls down the hill), to the crazy guy who works in the slaughterhouse. It's just full on hillbilly insanity. It's dirty, it's grimy. Really gives you the feel for how fucked up these people are.
I wouldn't call it the start of a genre though. You have 1922's 'Nosferatu' to thank for that. Chainsaw Massacre is in a genre all of it's own IMO. One they tried to ape in The Hills Have Eyes, and a few other movies, but failed at, because filmmakers don't seem to be able to make a psychologically disturbing movie without getting their balls out on screen and using FX to delimb people and saw their faces off in full color. There's no subtlety. I mostly hate horror films today.
I've seen the remake, and oh my god did they miss the point. On screen gore and dismemberment, dumb attractive teens being dumb and attractive. No misfit protagonists. Shitty movie. I might watch it tonight.
The sequels to the original were fucking retarded too.