The Mist is a good movie that could have been great.
The first element that lets it down was inescapable with its low budget: the special effects. The film is intended to be serious, meaningful and to capture real fear, and the acting and composition support this, but the creature special effects appear goofy and fake (aside from one effective scene set in a shadowy pharmacy). Goofy special effects can sometimes be visceral and potent (The Thing) or effectively silly (Return of the Living Dead) but here they just undercut the grim tone of the film.
The other was a missed opportunity rather than a failure of execution. Throughout the first third of the film there’s a deliberate theme of “taking sides,” where characters make choices based not on well-reasoned logic, but rather based on the person presenting the argument – whether they’re an “out-of-towner” or someone they have a history with. And those choices have inertia; it’s hard to change sides, to address prejudices. Sadly, the film doesn’t really address this, instead shifting focus to hysterical, religious mob rule. I would have much preferred an exploration of the inertia of taking sides; I feel it would have lifted the film.