Sean Collins thought the creatures weren't scary enough.
And the folks I went to see it with thought the whole premise was too goofy to take seriously.
For me, well, this is one of those movies where my appreciation for what it's doing is so great that I was with it all the way, even when it isn't 100% successful. It has flaws - a wobbly performance/characterization of one of the central roles (Marcia Gay Harden's), a final scene that works juts fine conceptually but plays out clumsily and ends up feeling like as much of cheat (although in the opposite direction) as the ending of Spielberg's War of the Worlds, and, yes, creatures that aren't that scary - but I don't really see the need to nitpick. I could and I probably would if it had been a big critical success like No Country for Old Men, but considering everyone else seems to be piling on (or just plain ignoring) The Mist it's more fitting that I step up to defend it.
Here's a little moment from the movie that kind of sums up everything I liked about it:
David Drayton - our hero - goes into the back room of the supermarket and sees, or rather, smells, that the generator is backing up and spewing fumes. He rushes over to turn it off - a good move - but, of course, that means the lights go out and he ends up bumping his head on an overhanging bar and tripping and stumbling over boxes on his way out. It's played as low key physical comedy and shows how despite our best intentions, the world gets in our way. David does the right thing, but that doesn't mean those boxes aren't there waiting to trip him up.
Actually, "waiting" is the wrong word, because that implies they have some kind of motive and, of course, they don't, which gets right to the heart of what makes this a horror movie: the complete and total indifference of the physical and natural (or in this case, extra-natural) to human intentions - whether those intentions are virtuous or vicious.
That also seems to be the point of the extended battle in the supermarket between the trapped humans and the little flying dragon thingies (I keep thinking of them as Lockheeds, after the X-Men character). The dragons break into the supermarket only because they're following their food source - those little bugs. For the most part, the seem to ignore the people, except when they're in the way. And, as it turns out, the ways the people come up with to fight the monsters - trying to hit them with flaming torches, shooting at them in a crowded store - are almost as dangerous as the monsters themselves.
I want to say that the fire "turns" on one one humans and burns him, but, again, there's no intention here, just indifference. They try to harness the fire for their own purposes, but their mastery over the physical world falls tragically short.
For me, that's where the horror comes in and that's how the movie earns its scares: not through fear of the monsters, per se, but through the fear of our inadequacy when faced with them and the fear that being forced to wrestle with the massive indifference of the universe will either drive you crazy or crush your hope for the future.
These aren't necessarily original themes, but the movie dramatizes them with what I thought was a nearly staggering amount of emotional force, which is, in turn, grounded in very specifically-realized details*. Though on a smaller scale, this made the movie feel a lot like one of Steven Spielberg's (esp. War of the Worlds). But it diverges from Spielberg in its pessimism and refusal to hedge its bets (as my friend Nick texted me: The Mist is "ballz to the wallz")**.
As for the goofiness issue: different people will bring different standards to the table, and, I've noticed, very few people are consistent about it. That is: some people will balk at taking stories about super-powered mutant heroes seriously, but have no problems with stories about the living dead. Other people might be completely down with the whole flying dudes in tights thing, but just can't believe that anyone over the age of 12 would be interested in stories about a teenage wizard. In general, I'm pretty accepting of any kind of fantasy element and while I recognize that it's pretty common for folks to draw a line somewhere or other, I can only just wrap my head around doing that.
In terms of The Mist, my guess would be that if you're at all resistant to the idea of taking a movie with creepy, crawly creatures seriously, it just won't work. I'm not sure this is the fault of the movie: the creatures are pretty nicely done, but they're also drop dead serious. There's not as much for goofiness as there was in The Host, say, or Gremlins.
*28 Weeks Later (my other favorite horror movie of the year) deals with a lot of the same stuff. It's a leaner film and is, I think, overall better made: very little of it doesn't work. Still, for me, The Mist packed more of an emotional punch, maybe because, at heart, I'm more of a Spielberg-kid than a Romero-kid.
**Nitpick: I think the ending works conceptually, but it doesn't quite play out in a satisfying way. It makes the events of the final sequence less a tragedy about the loss of hope and more about bad timing. My guess is that Darabont would have needed more time to really pull it off correctly. As it stands, it felt a little like the Mirror Universe version of the War of the Worlds ending - a downer, for sure, but not one that felt completely earned.