I did not expect Prometheus to be, nor want it to be, an "Alien prequel". It was going after bigger issues, and I think it succeeded in addressing a whole range of issues. More than anything else, to my mind, it was about the consequences of hubris, as personified by Weyland and apparently also the Engineers, and what it means to be human, as evidenced by David and Shaw.
These are the thematic aspects; also it is visually stunning, I saw it twice in IMAX 3D and it was like being there. Absolutely incredible.
Each time I see the film, I notice new interstices that link and foreshadow elements to come. And no it doesn't answer all our questions, although if you watch it really carefully, it does answer most of them.
Time will reveal it to be a work of cinematic genius, on a par with Blade Runner and 2001.