This movie is, in my professional opinion, the best movie of all time. You want to change my mind? You can't! You want to show me a movie that's better than this one? Buy it for me and I'll watch it and tell you what I think! Good luck!
On a more serious note, this movie is so incredibly important to me for all the reasons possible, and is definitely a highlight of the sci-fi horror genre in my eyes, even film in general. Every aspect of it is extremely well constructed and serves to shape the film into an incredible piece, such as its incredibly realistic acting, unnerving atmosphere, and writing that keeps the mystery and suspense of the story still present and strong even 40 years after its release (and after dozens of rewatches of the movie myself). Not to mention the horrifyingly stunning work Rob Bottin performed with practical effects, as well - the extremely unorthodox yet genius method of utilizing melted plastics, foods, sculpting, and elaborate puppets all mixes wonderfully together to create a truly incomprehensible and paradoxical monster. It tests the strategy, awareness, and synergy of all the characters to fight against this threat - the monster that is everything and nothing, everyone and no one.
Writing, acting, set design, score, practical effects - this movie hits all the marks and goes above and beyond for its potential. The writing takes not only the characters but the audience along with the mystery, respecting their intelligence and allowing them to engage with the movie passively by trying to decipher what's happening in the crew at any given moment. The tension and atmosphere heightens this further by increasing the urgency of the situation at hand, and all of this combines for an extremely immersive experience through its writing alone.
The atmosphere itself is expertly constructed by the score, set design, and most of all, its practical effects - all weaving together with deep, droning tones, desolate snowy fields, and disgustingly real and chaotic body horror to force a profound loneliness and urgency onto the viewer. No one is coming to help, and you are forced to save the human race from a threat beyond comprehension without anyone ever knowing. Thus, its horror persists in this reflection, tapping into the near instinctual fear of being alone and in danger.
Something notable about its writing in this fear and urgency and struggling against this threat, is how the beginning foreshadows the eventual fate of the American research facility - that being the Norwegian research facility and Lars, the last surviving man from the building. He acts as both a plot device and foreshadowing, though this isn't clear from your first and only meeting with him. His actions seem to be just craziness from a first glance, but further into the movie you slowly realize he was trying to save them all from their greatest threat. This is both shown from the simple connection to his shooting at the dog, but also through his one spoken line. He says, in Norwegian, "Get the hell away from that thing. That's not a dog, it's some sort of thing! It's imitating a dog, it isn't real! GET AWAY YOU IDIOTS!" This desperation in his words is completely lost on our English protagonists as well as an English audience, which only deepens the tragedy of his Norwegian and their English facility's slow descents into madness and chaos and death. This descent is further foreshadowed in Blair and Macready's exploration of the ruins of the Norwegian facility, giving them a glimpse of their future without them even knowing it.
If you couldn't tell already, this movie is fantastic. Its impeccably horrifying writing, immersion through its atmosphere and respect of the audience's intelligence, well-crafted score and set design, and its extreme use of practical effects to drive home the fact of how real everything is - all these elements serve to create a wonderful movie that is always able to be enjoyed, even as it ages, and even as you watch it again and again.
I personally don't believe this movie has any weaknesses, but some things that people find to be offputting about the movie are its slow introduction and the extreme amount and abnormality of the gore and body horror. Considering the nature of the film's premise, I feel that this is appropriate (and honestly makes the movie more enjoyable), but I can see how this might be overwhelming or superficial for a person that's not accustomed to the movie's elements.
There are also a few moments where, for the sake of showing off the Thing, the characters freeze or act irrationally. While this is technically an appropriate trauma response psychologically, in the context of writing fiction, it can feel frustrating and out of character. This movie is characterized by a cast of characters that don't simply run from their threats - they fight back, they strategize, and they do everything they can to ensure their survival against the Thing. Having these brief moments where they simply freeze like in most horror movies can understandably take away from the immersion (though I don't blame Carpenter for wanting to show off Bottin's work!). These moments are few and far between, though, and are usually compensated by other characters calling out their inaction and showing anger themselves. Thus, I feel this is a very minor weakness in the film.