Narm Charm: The whole point of the movie! Especially the ridiculous noises the spiders make! They roar, they hiss, and when they're hurt they act like cartoons! When set on fire they squeal, when squished they go "bleah", and you might even hear a "whee" or two!
Nightmare Fuel: Yeah, yeah, the first kill is done by a Pinktoe Tarantula, but imagine this. Your spiders have gone mad and one of them gets a hold of you. The rest are now free...and the last shot of that scene is the parrot letting out a blood curdiling shriek as a tarantula grabs hold of him.
Remember Gremlins? Where the monsters were causing death-inducing chaos across town? Even with all the goofiness, it's TAME compared to the rampage across town.
Still think it's still goofy? It is, but what do the spiders do with any humans they capture? Namely, liquifying your insides in an extremely painful and slow way. Don't believe me? The female Orb Weaver from one scene will show you for a moment...
Nightmare Retardant: Granted, it's by no means a straight horror flick to start with, but the goofy noises the spiders keep making can easily defuse the terror for non-arachnophobes.
The movie's first death is caused by a (regular sized) pinktoe tarantula, granted it's already supposed to be a giant mutant, but they used a regular Avicularia Avicularia as the killer spider. Pinktoes are generally known by hobbyists as one of the gentlest, most docile, most timid of tarantula species. It's the equivalent of making a movie about killer wolves and have the first death caused by a basset hound. Possibly justified, if the toxic waste also made the spiders more aggressive, which there is evidence for - most spiders, even the ones that don't regularly spin webs, are ambush predators and don't really chase down prey.