In mainstream low-budget porn, which comprises the majority of what's available through local retailers, online streaming video/clip websites, and DVD rental sites, the quality you're going to see is roughly that of regular (nonerotic) independent films, daytime soap operas, or daytime made-for-TV movies. These films feature simple sets, usually indoor filming in one location, standard harsh lighting, digital cameras (which are what contribute to their daytime TV look and feel), and barely present acting. Big studio films, by contrast, can be surprisingly complex in script, have terrific cinematography, and show sexy, believable actors, though the movies (and actors) have a purposely packaged and polished finish. Not surprisingly, the biggest studios have bigger budgets, access to better sets, actors who might have gone to acting school, writers with writing experience, and directors who are more likely to take their craft seriously. Some even use real film stock in their movies, which is costly but produces a higher-quality film, in look and feel. Sound quality, however, is generally poor, even in the big-budget feature films. The independents can have the same quality range, but they're less likely to be found in a neighborhood video rental shop; they can be found more easily online or in a sex toy or lingerie boutique.
Not all porn has a plot, and in all-sex DVDs you'll find that same variation in quality, from high-budget polish to low-budget rough edges. Many of these look like home movies, because that's what they are. The digital camera has made it possible for anyone with a little money and the right access to make and distribute a film. The medium- to low-budget DVDs will often contain extremely explicit ads for phone sex that are cheaply recycled footage from really old videos, with one come shot after another. Sometimes these ads are pretty aggressive, and contain scenes that might offend some people, such as gaping anal shots. Sometimes the immediacy of the ads' sexuality will make the films themselves seem anticlimactic; I know of several women who get so hot so fast by the shocking ads at the beginning that they claim they don't make it through the ads, masturbating to orgasm before the ads are finished!