Moon is something of a dying breed in the sci-fi genre. Where most science fiction films these days could be best described as enabling 'epicness', Moon gets back to the roots of science fiction with its small-scale scope.
What I mean by that is, compared to films such as Star Wars, Moon is very modest. It takes place on a rock humanity was able to reach in the 1970s, and it's set in what is only the near future. However, the setting is merely a backdrop for the real story. This film could have just as easily taken place in a desert on Earth or a remote planet or really anywhere that's suitably isolated. The Moon is the perfect backdrop, however, because of its proximity to Earth and ability to seem extremely far away but still be within reach. You can see the Earth in great detail from the Moon, so it seems that much closer. As a man who has been in almost complete isolation for three years, Sam Bell can see his goal. He can see his home, and perhaps that makes his stay on Earth's only natural satellite that much more unbearable.
As has been noted many, many times, Moon works better the less you know about it. If you haven't seen the trailer yet, don't watch it. If you have seen it, try and forget it (something I somehow managed to do). The less you know about this going in, the better. It is worth noting that even though the trailer seems extremely revealing, it's actually not. The things shown in it are actually misleading as to the size of their role in the film. However, even though it's not a huge spoiler, the elements revealed in it are better left unseen until you actually go and watch the movie.
That's all I'm going to say about it (what I've already given away may have been enough), so if you still want to see it I suggest not reading the rest of this article as it is going to become extremely spoiler laden. Perhaps my favorite thing about Moon was its complete willingness to avoid science fiction cliches. For a film that contains each of: space, cloning, a huge unethical corporation, a base on the Moon, an AI that runs the base and isolated man slowly going insane, it surprisingly avoided doing almost every single thing I was expecting. Early in the film it's revealed that Sam Bell has been on this base for three years with his only human contact limited to video messages from his wife and young daughter back on Earth (real-time communication has been disabled). The only things he has to talk to are his plants and the resident AI, GERTY, who isn't much of a conversationalist. As such, he's starting to hallucinate.
At this point I was expecting the movie to branch and become a twist ending affair, where large portions were hallucinations by Sam, but it never did. Likewise, once Sam's clone was revealed I expected some kind of struggle between clone Sam and real Sam where the clone wanted to replace Sam and take over his life. Never happened. In fact one of the most compelling things about Sam's reaction to his clone was the reaction itself. Rather than some huge freak out as most movies might do, Sam's reaction was simply one of a small amount of shock followed quickly by acceptance. He's been alone for three years, and now he has someone to talk to. Finally. He asks to shake the clone's hand and they play ping pong.
The clone's reaction is also pretty interesting. He finds the older Sam in a crashed rover and brings him to the base. He questions GERTY, who skirts the answer, but it's fairly clear he knows what's going on. Once the other Sam awakens the clone briefly tries to say he's not a clone, but ends up letting go of that argument since it's fairly obvious he is. Again, he could spend half of the movie refusing to believe he's a clone, even though it's pretty obvious to the audience that he is one, but the film never goes down that path.
The next obvious thing is if or not the original Sam is actually the original Sam. Was he first or is he in the middle of a long line of clones? This is held onto a little bit longer, however when he point blank asks GERTY if he's a clone or not, GERTY can only ask him if he'd like something to eat. It's later revealed in a number of ways, from GERTY telling Sam the process for a clone boot-up to Sam calling his daughter who is now 15 instead of 3 (meaning this Sam is at least the fourth clone).
Another large but seemingly accepted cliche in science fiction is the cold, unforgiving and very often totally evil (but logical) computer AI that controls a spaceship or base. HAL is of course the pioneer in this field, but he's had some good company over the years. It's something that's always sort of a given. Whenever there's an AI overseeing they tend to logically deduce that humans are a threat and that they should go against their programming and behave in a way that ends up killing a lot of people. GERTY more or less smacks that convention in the face. His programming states that he is to protect Sam and the eventually leads to him helping Sam discover the truth about the whole ordeal. He gives Sam the password to access the surveillance system (allowing him to see former copies of himself meet their demise at the end of their contracts) and goes as far as to wipe his 'memory banks' so he won't know anything about the previous Sam's escape from the Moon.
When the second clone Sam realizes he's a clone, and realizes that he was awoken or thawed very quickly he surmises that he was stored somewhere on the base and that there are probably a bunch more. He spends a lot of time searching for a room full of clones and it actually ends up being found by the first clone later on. This is something that would have been a major reveal in any other movie, but Moon treats it simply as part of the story. The viewer is left kind of going "oh, yeah that makes sense" instead of attempting to being shocked into a room full of clones which is something that's been done enough in science fiction that it's not really a shock at all at this point.
What you end up with as a result of this is a very solid and almost self-aware movie that is not actually about cloning or space or isolation, but about all of these things. It's something that very much feels like it should be a short story adaptation even though it's an original screenplay. It's very localized, there aren't many characters, and it's almost entirely self-contained. It also intentionally leaves a few things open-ended, so there's a lot of room for interpretation. Why did Lunar Industries clone Sam? Was the original Sam ever on the Moon, or did he somehow loan his DNA to Lunar Industries? Why do the clones only live three years? None of these really need answers as the movie works perfectly without them unanswered, and they serve to spark some interesting discussions after the fact and certainly keep people talking about it. A sci-fi movie where you discuss the plot and don't talk about the action or special effects? What a crazy idea.