Slither is a horror comedy that doesn't really have enough horror or comedy to qualify as one or the other. It has one scene that is exceptionally good, any number of zingers that work, but very few real scares and not enough humor to maintain the movie. In addition, the script does not focus on the hero and heroine, and goes off kilter in several places.
A major failing of this film is that it introduces and then leaves its hero (Fillion) to follow Grant Grant (Michael Rooker) as he is first introduced and then becomes the monster. This whole part of the film drags - Michael Rooker's character isn't that interesting to us as a person, and watching as he goes through a series of motions while acting in the monster's interest might be interesting if this was Grant - Portrait of a Man Turning Into A Monster rather than a horror-comedy alien invasion movie. In the final analysis this movie's problems are in the script - it isn't that important to the audience how the monster acts or propagates. The purpose of a horror-comedy is to get the heroes backed up in a corner with shotguns and then throw bugs at them, with them cracking wise every time something frightening or disgusting happens. Instead we get an exploration of the alien's habits and tactics that just makes this part of the movie drag. The ostensible heroine (Elizabeth Banks as Starla Grant) is more central to this part, but nonetheless I felt the movie had left its narrative track, unless it planned on following Grant Grant all the way to the end.
When Fillion and his posse finally confront the alien the movie does begin to cook, but once again the problem is in the script. By this point that audience knows - and the characters should know - that Grant is not just suffering from some disease, and act accordingly (shotguns) - instead they continuously parley in the face of increasing evidence that this is not something that "let us get you to a hospital" is going to help. Although their reactions might have been human and real, these are characters in an action movie and simply should have done what the movie promised - delivered action. A lack of action scenes in a movie with as few ideas as this is a great failing.
*** SPOILERS AHEAD *** After the first confrontation and the bursting of the alien larval sack (a minor character and perhaps the best scene in the movie) the script once again betrays the movie. At this point one of the characters is almost taken over by the alien and develops an insight into the alien. The writer-director (Gunn) chooses as this character a completely new character, rather than one of already developed minor characters. Why? Why did he need to introduce a completely new character more than an hour into the movie that becomes central to the plot? By the time this character is attacked, we know hardly anything about her and could care less about her, even though she is a winsome teenage girl in her bath. Had Gunn decided not to use this character and just used one of the established minor characters, he could have completely avoided introducing her family, and saved time and money. Furthermore, the hero and heroine would have been filled in on the alien's plans without all the additional characters, and could have gotten around to blowing away aliens sooner and with more vigor.
My last criticism is based on the movie's look. Gunn is primarily a writer, or maybe it was budgetary constraints, but this movie looked ugly and uninteresting. Most of the action takes place at night in woods or on a field, and the screen simply looks drab. The sets in Wheelsy (the fictional town where the action takes place) look cheap. The whole movie looks cheap. Box Office Mojo states the films' budget was $15 million, newspapers say $29 million, and considering they didn't use any name talent, I would say the money did not show up on screen. The monster is just repulsive, and rarely looks deadly.
The last criticism is primarily based on the reality of the character's actions. By the time Fillion and Co have begun hunting Grant/the alien, one woman has disappeared and Grant is known to have been mutilating animals. At this point I was expecting the FBI or at least the State Police to show up and take over from the hick Sheriff. A woman has disappeared and likely been murdered, and a local has been acting psychotic. Time to call the authorities. But basically I was hoping that would happen because I just wanted some characters who would show up and ACT.
Although this movie is ostensibly a horror-comedy, the movie it bears the most resemblance to is Dreamcatcher in terms of monstrous invasion and the type of monster and its intentions. Whereas Dreamcatcher had much bigger problems with story (especially the entire Morgan Freeman subplot) and particularly the ending, in many ways it was stronger, primarily because the main characters were stronger, but more importantly because it looked beautiful. Although that may be anathema - preferring the movie that is weaker in general plot and structural spine because of production values - that just shows you how uninteresting I found the look of Slither.