I'm not disappointed by that at all. Prometheus lifts a small tip of the veil surrounding the Engineers but a whole lot more about the Alien creature, funnily enough. It expands dramatically on their life cycle and makes them, in my opinion, infinitely more "alien" than before.
The fact that none of those questions are answered is what's so great about Prometheus in my opinion. It gives us the Engineers and expands massively on the universe... but, yet, tells us absolutely nothing. It raises more questions than it answers, but it raises much bigger questions than before. The questions used to be "what are the space jockeys?" and "did they make the aliens?". Now the questions are "why did the Engineers make us?" and "what did we do to deserve their wrath?"... along with "what the hell are the Aliens really?", which is the greatest possible thing it could have done.
Prometheus also leaves plenty of wiggle room for a sequel. In fact, it splits the story into two branches: one has already been explored and deals with the bio-weapons they created: the Aliens. But when you look at the other one that is yet to be explored and developed (the origins of the Engineers, their motivations and their true nature) it seems subservient. Less consequential. The franchise we cherished and loved all these years is just a part of a larger, overarching epic about humanity's creation and our unfathomable makers.
In fact, I'd love to see a sequel to this. Prometheus sets the stage and raises the questions... questions so big they deserve their own movie, in my opinion.