Hostel presents a well-done study in audience manipulation, like Bunuel's Land without Bread (Las Hurdes). I am normally not a proponent for torture in general (an eye for an eye and the whole world will go blind), but this film made it very clear that if someone tortured a friend or member of my family, I would feel a great need for righteous vengeance. The end of the film hints (read the last sign) that this story would be told again in a court setting. How would they/you explain their/your actions? Would justice prevail?
In a different vein, the film also probes (lightly) the use of language to both bind and differentiate humans. There are no subtitles, but significant lines are spoken in Germanic and Slavic languages (Dutch?, German, Russian, Croat?, and a smattering of other languages) . Either you speak these languages or not, driving home the existence of language bridge or large gap between people and peoples (which parallels the film's core of people paying to torture people from different countries)
Of course, it is possible to skip past this cerebral poo, and just watch a fun horror film. Which is probably why this film is doing so well at the box office. ;)