Their gangly legs carry them at record-breaking speeds (1.88km/h according to Guinness World Records) that startle even the calmest humans. Their pedipalps on the males are mistaken as long fangs, and cause frequent misidentification as tarantulas.
Their hairy bodies obscure their segmented bodies, causing fear for what could be hidden beneath. And those beady little eyes.
While just about everybody has gotten spooked by one of these running in front of the tv at night, they aren't deserving of such fear.
Despite their fearsome looks, they seldom bite. I've interacted with these spiders for years, and only the appearance of food would cause them to strike (which is hard even for a human to imitate). While some people claim that "they ran straight towards me!", it's much more likely that they ran in a random direction, which happened to be where they were standing. Their beady eyes only sense light/dark, and are incapable of determining the direction of a threat. Their reflex is to run for cover.
They build funnel webs, and pretty much stay put their whole lives. The hairs are their body allow them to sense vibrations in the air or their web to give them instant input on what direction prey has landed and how large it is (whether it deserves a hasty retreat, a quick bite and retreat while the prey succumbs, or to grab and hold). Despite living in the dusty corners of most basements, spiders are generally fastidious creatures, staying well groomed to keep their sensitive hairs in their best performance.
The males (pictured above) once mature will seek out females, and these are the ones that typically scare human members of the household into dropping a book on them, or for the more ecologically sensitive, an empty glass.
The image above and below is that of a mature female. You'll see the difference in the appearance of pedipalps, which are the short leglike objects to each side of its mouth. In the male, the pedipalps look swollen, because they hold the sperm packets that it intends on using to fertilize the female.
Both of the photographed specimens I found in our storage shed, living next to each other. While giant house spiders wouldn't intentionally share webs, sometimes they start out small, but their webs encroach on their neighbours, and they tolerate each other's existence as it's riskier to venture out on their own to seek a new home than to stay put. The enemy you know vs the one you don't...
While many call these hobo spiders (which do have a higher notoriety for their bite toxicity), the ones I photographed do seem to be in fact giant house spiders given their large size (in fact, their large size seems to be the main reason why hobo spiders are, in this part of the word, rarer). Historically, both hobo spiders and giant house spiders were once considered part of the genus Tegenaria, but advances in spider identification resulted in the Giant House Spider split into species Eratigena atrica (the astute would notice that Eratigena is an anagram of Tegenaria) Size of course isn't a reliable indicator of species; more reliable would be examination of the shape of the male pedipalps or the sternum on the female.
While one might think that by their sheer size that these spiders are at the top of their arthropod food chain, wandering males frequently find themselves ensnared by cobweb spiders who are usually only a fraction of the size. Home turf is a huge advantage for web-building spiders.