It's weird how so many apparent fans of Alien get up in arms about this film asking more questions than it answers. Alien answered basically nothing about the questions it posed, and that's actually one of the absolute great things about it. To me picking on Prometheus because of this is just looking for problems to explain unarticulated reasons you didn't like it.
Alien:
Egg -> Facehugger -> Chestburster -> Adult Xeno
Pretty simple. "Aliens" is mostly the same, with the addition of the Queen.
Questions about the Xenomorph:
"Acid blood? How does that work?"
"How does it grow into an adult so quickly?"
"How were the eggs made?"
Questions about the origins/mystery:
"Where did the Derelict come from?"
"Who was the Space Jockey, what was he trying to do?"
"Where are the eggs from, what are they meant for?"
None of these questions interfere with the logic and plot of the movie. The characters bring up a few of them, but ultimately decide that the answers are irrelevant to their plight.
Prometheus:
Black Goo + Engineer -> Dissolves into DNA?
Black Goo + Worms? -> Space Snakes
Black Goo + Vodka -> Bloodshot Eyes, Massive Headache, Death by Fire
Black Goo + Human + Sex -> Squid Baby -> Giant Squid
Giant Squid + Engineer -> Retarded Xenomorph
Black Goo (or snake acid?) + Melted Helmet -> Super-strength human hell-bent on killing redshirts.
Pretty convoluted.
Questions about the Black Goo:
"Mutations? How does that work?"
"How does the squid grow so quickly?"
"How is the black goo made?"
"Why does it behave so inconsistently?"
"Why did the Xenomorph come out so large?"
Questions about the origins/mystery:
"Why did the Engineers make humans? Why did they stop visiting? Why did they want to kill us?"
"What did the cave paintings really mean? What did the space mural signify?"
"What were the Engineers in the hologram running from? What killed them?"
"What was the giant head for? Or the green crystal?"
"Why was an Engineer still alive? Why was he asleep, why didn't he wake up earlier?"
"Why did he try to kill the people that woke him up?"
"If there were other ships on the planet, why didn't the Engineer go there after his first ship crashed?"
"Did Shaw find more Engineers at the other ship? More black goo monsters?"
These questions aren't just backstory outside the scope of the plot (like the Alien questions). These are vital to the events in the movie. They're the basis of character motivation.
Ridley didn't care where the Xenomorph came from, she just wanted to kill it.
Weyland wanted answers, but just got a punch in the face.