Same diff, really.
It absolutely destroyed everyone's expectations at the point of release. It broke the 'enough' threshold without hitting the 'silly' wall. 'Cause that's the thing. If the audience says 'Alright, thanks, that's enough', f**k them, edge over the line just a little more. That's what Alien did, and it didn't need gore to do it; they just used ridiculous ideas and creepy creature behaviour.