Examples include the intense focus on competition and prizes (I didn't think it was a good idea for YC Hacks to explicitly advertise the prize of getting an interview), the fact that many hackathons now have applications processes where more applicants are rejected than accepted (so that the net effect is more exclusion than inclusion), and the general vibe of having to power through the night with redbull while cranking out code.
Most hackathons I've attended I've usually gotten this exclusive, rather than inclusive vibe. From my own experiences, the most exclusive hackathons have been the larger, more monied, more applied-to ones that have more applicants than attendees. The most inclusive hackathons have been smaller, intimate hackathons where the problem has been not having enough attendees!
EDIT (reply to below): I'm referring to the general feeling of inclusion vs. exclusion (of newcomers, people not sure if they fit the mold of <quote>hardcore<unquote> coders, etc.). The next step is to argue that exclusion discourages minorities from pursuing the field.