There's a founder effect, but my contention is that at a certain size, the founder effect is swamped by the Here Comes Everybody effect, or how Reddit turned from being a techy, nerdy site with site-wide in-jokes ("The narwhal bacons at midnight") to being a site you'll get a markedly different impression of if you land in /r/SubredditDrama versus /r/news versus /r/Conservative versus any of the niche fan-subreddits for different things.
The problem is that people do see a subset of a whole site, or a subset of a whole city, or a subset of a whole country or ethnicity or culture, and come away thinking they've Seen The Thing. Take that far enough, and you get nationalism, or racism, both of which spring from the underlying notion that broad groups of humans can be classified minutely as if they were species of insect, as opposed to being large numbers of endlessly complex individuals, each with a distinct life and set of experiences.
