HERE BE SPOILERS:
A “problem” with Alien, if you can call it that, is that there is no way to recreate the experience of seeing it in the theater when it first came out. No one had heard of Sigourney Weaver then. The other actors in the film had varying degrees of fame-- Veronica Cartwright had been acting since she was a child: she was Violet Rutherford in Leave It to Beaver, and had appeared in Hitchcock’s The Birds. John Hurt was still riding international glory from appearing as Caligula in I, Claudius: he had been famous in the UK for a long time, and now was famous all over the US.
So, what happened in the film that was shocking, and gave the viewers an anything-can-happen feel, was that the characters were killed in reverse order of the fame of the actors playing them. That was completely counter to expectations. Usually the unknown in a film like that is the redshirt, and the most famous actor is the one who is left at the end.
No one thought Ripley would survive at the end, and the tension of the last ten minutes, when the computer is counting down “T minus n minutes,” was unbearable. It wasn’t* if the alien would get here, but when*, and how. Until the credits rolled, no one really thought Ripley was safe.
If you see the film now, even if you manage not to know how it ends, Sigourney Weaver is still the most famous person in the movie, so it’s not surprising that she is the one alive at the end.
Anyway, The Thing had nothing like that kind of tension. Also, while it’s true that Alien is loosely based on a movie called It! Terror from beyond Space, It! isn’t a very good film, and Alien doesn’t resemble it that much. The Thing, on the other hand, is a clear remake of a very tight film from the fifties, that some people don’t think needed remaking.
That last bit is mostly opinion, but it’s one I heard a lot around the time the films came out. Of course, I had seen the original Thing before I saw the remake, and I didn’t see It! until years after I had seen* Alien*, and that probably made a difference.