The atmosphere is a key element, also. I'm surprised nobody brought up Clint Mansell's hauntingly excellent soundtrack (I've been playing "Welcome to Lunar Industries" over and over, maybe I'll go listen to it again now).
It's more of a human reaction. Sure, the obvious conclusion is in the back of the Sam Rockwell's character's mind when they first meet, but it is a rather absurd one. If you were suddenly presented with another you, how would you react?having dense characters stretch out what is blindingly obvious to the audience.
I recall having a bit of the same reaction the first time I saw it, last summer. But it's important to remember--and I noticed it more keenly after I watched it tonight--the entirety of the Sam on Sam action takes place in a twenty-four hour period or less.
Given the time constraints, I think they figured everything out pretty quickly. They certainly got more done than I did today.
The retro feel is intentional; it's sort of an homage to Silent Running and such (just as GERTY resembles a certain computer A.I. we all know and love). I found that pretty cool, but then, I like Silent Running.[/quote]although the interiors felt blocky and even somewhat retro.
I will say that it scared the crap out of me when some reviewers compared Moon to Silent Running, and I went and watched Silent Running. Different strokes for different folks, I guess, but my God, I don't think I've ever seen a movie that bored me more.

It is similar, though. However, 1)Sam is actually a likeable character, unlike Ra's al-Ghul there; 2)Sam is not really alone, so there's character interaction; and 3)GERTY is much more interesting than the dumb (in both senses of the word) bots from SR.
GERTY is wonderful. I really liked Jones and Parker's take on its AI--well-spoken but somewhat impassive, sometimes to the point of being kind of stupid, but capable of feeling, compassion and a conscience. The development of GERTY's fledgling morality, and his sacrifice at the end, is as interesting in its own fashion as Sam's interaction with Sam.That attitude does partly inform the film as it plays with some sci-fi conventions with a degree of wry humour; GERTY is the inversion of HAL, which is why he has both HAL's eye and a smiley face (and so on).
As for what else Mr. Roman's missing, or at least what else I thought was fuss-worthy about Moon--
The subtleties of the Sam/Sam scenes are the real meat of the film. Rockwell doesn't play one character, he plays two--effectively identical twins with very different life experiences. The older Sam is a much different character than the younger Sam, and their interplay is fascinating.
The atmosphere has been mentioned. It's true that it's tremendously effective. The Mansell score is fantastic as usual. Jones' direction is also some of the best I've seen since, well, a Kubrick film.
Regarding the plot, not every plot needs a twist. Moon's a tragedy--one man, or rather two men who discover their lives are lies, and how they react to that revelation. The twist, if there is one, is that Sam's a clone. It just happens to come in the first twenty minutes. The film is, in part, a struggle to permit the Sams to use the tiny fraction of a human lifespan they've been given to enjoy a real life. It's tragic because even if they did manage to escape, that's not going to happen.
Their utmost desires are to go home--but it becomes clear to the older Sam, eventually, that he doesn't have one. His wife's dead--and of course, she was never really his wife in the first place. The real Sam Bell is still alive.
The older Sam recognizes that more than the younger Sam. The younger Sam is motivated more out of a sense of fairness--which is why he is ready to stay for the three years in order to carry out their ruse, and let old Sam "go home." Old Sam, by contrast, realizes the futility--and this is, I think, part of why he changes young Sam's plan, and sacrifices himself. Another part of that decision is the moral choice the older Sam makes, to refrain from taking the life of a third clone so that he and the younger Sam can live. (The last part, of course, is that it's not very sensible for the man whose organs appear to be liquefying to go home just so he can die a few days later.)
So there's a lot of depth to the plot, with the overriding theme of what man does in the face of certain, soon death, and pointless existence in the meantime. At least that was my take on it. It's certainly not just Sam Bell standing around for two hours talking to himself (ala Silent Running

In short, Moon was the best film to come out in 2008 (as technically it did). It was the best film of, hell, the past ten years, and probably the next ten years. The last movie to come out as good as Moon was probably The Truman Show.
Interestingly, they're both sort of stories about isolation and abandoning that isolation to face the frightening prospect of a world they had no stake in. Moon had more menace to it, however--unlike the Truman Show, it didn't obviously require the character to make a difficult choice about the safety of the world they thought they knew, and the wonders and dangers of the real world. But then, Moon's premise permitted Sam, when faced with the analogous choice, to take both options--old Sam bowed to futility, while young Sam carried on the struggle, and did so--perhaps this a little overapt--in his name.
On a minor note, I could see someone saying this about District 9. D9 is good, but I wasn't awed. It wasn't stupid, but neither was it particularly brilliant.
Certainly, it was no Moon. (And neither was Avatar! Okay, Pandora was a moon. But a Moon that doesn't have Sam Rockwell on it isn't nearly as interesting, even if it does have flying waterfalls and blue furries.)
P.S.: Mentioning that Moon has clones in it is not a spoiler. It's the premise of the film. It's like complaining that someone revealed that Bruce Wayne becomes Batman in Batman Begins, or that Jesus dies in the Passion of the Christ, or that the aliens are hostile in Independence Day.