It's not a terrible film. Not by a long way.
It's a difficult film. It's risky. It's not entirely successful.
But terrible? Wisecracking CGI Superheroes Episode Number Actually I've Lost Count Now is a terrible film. (One of a long, long series.)
Men With Guns and So Very Much Shooting is a terrible film.
Too Many Explosions Looking for a Plot That Isn't All About Running and Shouting is a terrible film.
Dune was one of many drug-assisted weird things that fell out of the 1970s and never quite left. It's not a terrible film - it's a museum piece.
Minor point - whilst Lynch's work certainly frequently looks like it was drug-assisted, Lynch is actually not at all a fan of drugs as a creativity aid:
http://www.goodreads.com/quotes/498489-we-all-want-expanded-...
He just has a remarkable vision and ability with the weird.
That's a really low bar. A better comparison would be the three-part Dune miniseries that came out in 2000, one in which Lynch's version doesn't fare very well.
For example, watch the harvester-eaten-by-worm scene [1] [2]. Which of the two has stood the test of time? I could watch the 1984 version over and over again. Everything about the 2000 version - even the timing of each shot - is uninspired and dated, almost like a Westwood Command and Conquer in-game video. Watch how much more engaging the storytelling is in 1984 in this essentially dialogue-free chapter.
Lynch's version, for all its flaws, is still watchable.
The Lynch movie has, like most Lynch works, a strange atmosphere that in this case helps highlight the changes that Paul is going through as his consciousness expands.
That said the children of dune miniseries fares much better, but maybe because there isn't anything to compare it to.
The book heavily references the middle east, and oil dependecy.
The movie has zero, or at the least, very little visual symbolism. Especially if compared to Lynch's other work.
Drug assisted is the last thing I'd say about either the book, or movie.
I dunno about the movie, but I have heard sometimes that Herbert was quite fond of helping the creative process by those means.
http://dailygrail.com/Shamanism/2014/7/Magic-Mushrooms-were-...
A lot of the criticism misses the mark. I can try and find alternative explanations. The "outdated" computers? The Butlerian Jihad caused that future world not to build anything more complicated than a light switch. The unique worlds, monochromatic and with their odd uniqueness? Visible signalling of these being different cultures that survived by virtue of their unique characteristics. Unlike the "bugs vs Marines" of a lot of space opera, the Tleilaxu, Guild, Bene Gesserit and Harkonnen are all fundamentally different, their differences giving them a survival edge, influence and power. Black desert suits? Maybe the filters need solar energy to run. Erotic Bene Gesserit intonation? Have you read the later books?
This was the only depiction of Baron Harkonnen which I thought truly captured his sadistic genius. And the blocky shields are so much cooler than the form fitting transparent bubbles of later Hollywood SF (e.g. Star Wars), also explaining the difficulty involved in going slow for so long when trying to penetrate them (because the space between shield and human is so great). Or what about the worms, which despite (or perhaps because of) the sorry state of 1984 CGI somehow were just as immense as I had imagined them reading the words. There was this alien feel to all the vehicles, especially the ornithopters and the harvester carrier, which has only been recently replicated in the seemingly nonsensical vertical ovals of Arrival or the glaring contrast between earth and ship in District 9. This was definitely the artistic generation responsible for Brutalist architecture, which is not always fun to live in, but has a thousand times more soul than the International Style condos copy/pasted across all of the world's megacities today.
Lynch's movie was glorious precisely because it freed itself from the constraint of present day science and became a standalone creation from the inspiration of the source material. The other interpretations did not create that sense of awe. I found the miniseries particularly boring, a kind of BBC period drama feel instead of being transported into a completely new, intense world. What makes Dune great as a book and as a 1984 movie is precisely that it frees itself from "realism" and suspends disbelief in just the right way for telling a great story. Might as well complain about X-wings flying like F-14s and making sound in vacuum...