As always, IMO...
Okay, as usual, I am not any kind of authority on this subject. I wouldn't call myself a "horror movie buff" or whatever they call themselves nowadays, but I love horror. My first horror movie was Alien, and I was totally obsessed with it and had the toy.
I was very young and it was on cable. My mom covered my eyes in some scenes, but I remember that I was intrigued by the "space jockey" grossed out by the "facehugger", bothered that it got into Kane's helmet, afraid to see Kane die horribly and I was really scared of the "chestburster". We know what it grows up to be, but good lord, "Kane's son" is alarming.
I think the colors of the facehugger and the chestburster were what got me. I am not saying that I liked the way that Alien made me feel as a kid, but I was obsessed with it.
I don't know if it was my very young age or what, but no other movie I have watched has had that same effect on me. Sure I have been freaked out by a few other movies, but not to the level that Alien got me. Not to the point of obsession. Aside from my age being a factor, I wonder what else caused this.
Reveal/Know versus Suggest/Imagine
Alien scared me because I only got quick glimpses at the creatures and, there was plenty of screen time before Kane died, there was a lot of time to be puzzled and tension to build up. I didn't know what to expect. Ignorance is not bliss (well, ignorance of your own ignorance may be bliss). Not knowing can be a terrible thing that every single human being can relate to.
The Alien was only fully visible at the end of the movie, but even then, it wasn't clear. If the creature had looked "man-in-rubber-suit-like", the ending could have ruined it, even if it were not clearly visible, I would have seen a man in a rubber suit. Great effort was put into making the creature not look or move
* that way.
In the rest of the Alien movies, the creatures are not explained scientifically at all. They are called "facehuggers" and "chestbursters", etc. There is no mystery, no subtlety to them. I guess it is a result of marketing ALIENS toys to kids, but that sort of thing ruins it. They are given full display and in some cases, look like a man-in-a-rubber-suit or they look CGI.
I played a little game called
Call of Cthulhu for years. A role playing game set in the 1920's in H.P. Lovecraft's world of ultimate cosmic horror and the fragile human psyche. We had a lot of fun, but even though we were not being too serious (party of 6 with pizza) there were times when I got a little freaked by the combination of the story that was being told, a brief description of a something my character perceived and the music the keeper (dungeon master) played and not one time did I see a picture of a monster we encountered. The only picture I had was of my character~a picture that I drew on the character sheet and my imagination.

I think that another reason I became more obsessed with Alien is because the setting, which is set some time in the future. Is it 50 years? 100 years? Is America still around? Are there other, less hazardous life forms in space? These questions are not addressed by the script, but they are addressed by the crew of the Nostromo's patches, the back stories that were developed for them and the elaborate sets. The perfect amount of back story was highlighted in the script, just enough to make the characters real and not just an infodump for the writer's vision of the future. Little information is given that isn't necessary for the story to move forward. [Believe me, I do know that there is an actual Alien Universe Timeline!]
Artistic license - Biology and Body Horror
I wasn't a good biology student. I had to dissect a pig fetus brain to get extra credit enough to pass the class, but even I know that you don't just get some goo on your skin and undergo some huge metamorphosis unless the goo is from John Carpenter's "The Thing". You
might catch a disease from contact like that, but your genes are pretty safe, I'd bet. I am rather ignorant, though so you can sell it to me with a great story, pseudoscience, atmosphere and all that. Heavy on the pseudoscience and go lightly with the special effects especially if they have to be CGI. Build it up with tension, big words, character development and such. Many other ideas seem to stem from a more juvenile, gross-out kind of mentality, which is fine (I have a juvenile side), but it isn't horror; it's pointless, confusing, cheap, schlocky CGI effects at worst and simply pointless, confusing and (especially unintentionally) funny at best. Either way it turns out, I don't have any desire to watch it a second time and it remains utterly forgettable, unlike Alien was for me.
I just browsed through so many horror movies on Netflix that involve "pretty" people and disease, zombies, being infected, things coming out of people's mouths, rotting flesh and black veins. People criticize "Alien knockoffs", but there are not nearly many as there are zombie/apocalypse movies involving basically "cooties". Most viruses just make you sick and maybe kill you. Viruses don't transform you into something. They are parasites that use your body to reproduce. You may spread the virus before you start showing symptoms. Disease may make someone go insane or act strangely, but the cause, be it a parasite, an infection of some kind, genetic defect, organ failure or whatever it isn't beneficial and it doesn't make you superhuman, able to run, jump or physically assault someone and eat their brains, they weaken and kill you. I don't want or need to watch movies about people that are dying from a nasty disease unless there is a lot more to the characters. Another thing about diseases is that they are usually rather slow about killing you. Of course, not all diseases are slow acting, and some of them can kill you rather suddenly without much warning, but usually this is something like seizures, high fever, coma, heart attack, liver failure or something. I am probably forgetting something, but I don't care enough to fully research this thought out. It's just tired. I am not writing this to criticize zombie films, just to encourage people to maybe try to do something different than: A mutated alien virus changes people into ravenous monochromatic CGI monsters that attack pretty people and give them gory head wounds or infect them with mouth rot, filmed with a shaky camera in poor lighting and the only way they can be killed is with a closeup of their head being shot, actually blown off, split or chopped off.
Another thing i'd like to add about body horror is obsessions with things going inside people's mouths or other orifices and breeding. I get it, it is not a comfortable thought, but it has been done. A lot. Why keep on doing it? It is just stealing from Alien or an Alien ripoff. It isn't horror anymore. It is just kind of gross, especially when it is shown with CGI proboscises squirting eggs into someone. It's just basically alien porn. If you must keep making this kind of movie, at least make it funny or something, don't take it seriously and don't expect it to be scary in the least. It's just a little gross and that's all. IMO.
The Thing
I didn't watch John Carpenter's "The Thing" until years after it was released on VHS. I went to "Butch's" "Video Park" to rent This movie. The scene with the dogs nearly traumatized me in a similar way that the Alien movie did and, although I wasn't exactly obsessed with the movie, it remains in my top 10 at least. You all probably know that the effects were all practical; there wasn't any computer generated scenes and the biggest trick they pulled on us was some stop motion animation. Like Tommy Boy, Zoolander, Ace Ventura, Pee Wee's Big Adventure, Airplane, Naked Gun and many others, This movie goes from one of my favorite scenes to another. I can't really pick. You can pick up on the story of The Thing pretty quickly and arguably, the story is pretty scary: Some people in the arctic find something under the ice that wakes up and takes over people's bodies and absorbs them into it's own body and can mimic any life form it has absorbed. What makes this story good is the way it is filmed, the awful looking transformations, the creepy mood, the paranoia, pacing, the music, the sound effects, all these play an important part in making the movie work. If the transformation scenes had been computer generated, the movie would still be good, but personally I think the in camera effects, puppets and makeup are what make it better because their rubberiness, or "fakeness" has a sort of
uncanny valley effect that clean "perfect" CGI can't really reproduce IMO. This makes them look more freaky than if they were perfect computer models of the actor's heads.
Now in the below pictures, you can see a striking difference. The top picture, from the prequel, is computer generated and the bottom picture is from the defibrillator scene in John Carpenter's The Thing.


The movement of the creatures is much creepier looking with good puppetry. It just looks like a computer game monster with CGI. Now, with the prequel, they got the sounds right, they are pretty scary, and the gaping mouth is nasty looking too, but it doesn't look nearly as repulsive as the original work. The prequel made the creature more beefy, strong and clumsy, while the original made the creatures seem extremely hazardous, but feeble and almost insect like at times, especially with some of their sounds-definitely alien. The transformation and/or absorbing scenes were also very bloody, drippy and gooey in the original, making them quite sickening. There were green pustules bursting, pink and yellow dripping slime and other vomit inducing things that really made it horrific to look at. Disgusting at the very least. Now it is just a computer game monster.
*An Idea for Alien Movement
Japanese Butoh Dance - Not sure if it has been looked at for a movie monster's movement style, but it should be if it hasn't. Imagine a character in a movie becomes obsessed with a mysterious book and unwittingly (or not) summons a spawn from hell (or somewhere else) that comes out of their closet like the dancer in the below video: