I think the "issues" category on the geek feminism wiki does a good job of summarizing the "female-unfriendly" things[1]. Objectification and a "sexualized environment"[2] is what the poster refers to can be common on other sites. Sexism[3] and essentialism[4] is something that generally takes the form of attitudes or opinions, and therefor is less obvious. I see this here on HN in various forms e.g. this comment[5] saying "women are less likely than men to engage in deeply focused solitary activities". "Othering"[6] is something you can see even on this page.
[1] http://geekfeminism.wikia.com/wiki/Category:Issues [2] http://geekfeminism.wikia.com/wiki/Sexualized_environment [3] http://geekfeminism.wikia.com/wiki/Sexism - http://en.wikipedia.org/wiki/Sexism [4] http://geekfeminism.wikia.com/wiki/Essentialism - http://en.wikipedia.org/wiki/Essentialism [5] http://geekfeminism.wikia.com/wiki/Othering & http://en.wikipedia.org/wiki/Other#The_Other_in_gender_studi... [6] http://news.ycombinator.com/item?id=1550429