Why "Halloween" 1978 is a classic?

Compared to the contemporary slasher films ( with blood and guts displayed viscerally ) and even compared to John Carpenter's other horror classic The Thing, 1978's film Halloween is rather un-scary in comparison. Yes, it is a slasher, where a lot of teenagers die. Yes it has a lot of disturbing ideas and a lot of rather good cinematic tension. But it is weak in the blood department. Which begs the question: What's so special about this movie?