The dialogue is universally awful
I guess that's why it's so universally quotable and a part of the pop-culture zeitgeist, right?
There's a reason why people can quote 'Aliens' left and right, it's because it's well-written dialogue.
excessively wasteful - which surprises me as he came from the Roger Corman school where making the best of what you've got was a necessity
I'm not sure how you can say this. 'Aliens' had a really small budget, and he made a massive swarm of Aliens by using 6 people in suits and creative editing. 'The Terminator' was barely completed because he ran out of money. The shot of the Terminator getting its head crushed in the press is literally spraypainted foam, tinfoil, a christmas light, and Stan Winston blowing cigarette smoke.
i guess it's dangerous to suggest that Prometheus is going to be the best sci-fi film since Blade Runner
There have been plenty of great sci-fi movies since Blade Runner, most recently 'District 9'. Hell, 3 of the great sci-fi movies since Blade Runner were directed by James Cameron.
(Aliens, Terminator, T2)
I'm not even sure why we're discussing Cameron here - the point is that saying "It's Ridley Scott, of COURSE it's going to be good!" is a statement that should give people pause.