Alien is by no means a deep movie. Prometheus, however, started off as an ambitious, intelligent, plot-driven sci-fi. And then, at some point, all traces of intelligence just evaporated and the movie turned into a dumb syfy horror flick.
It was like watching 2001
for about half an hour, than suddenly switching to The Thing
The visuals and the music are phenomenal, but they simply cannot redeem the terrible writing.