* Bots and sock puppets pushing agendas
* Reddit counter culture movements and backlashes. Seems to be bipolar with pro China / anti China every day of the week.
* Reddit nice guys who have account names like "Iliketitties" or something really depraved. Along with all the other hypocrites
* Reddit Admins and big personalities driving content trends on the site e.g. (/u/gallowboob)
* Recycling of posts / knowledge, since there are always new people joining. You get things like "TIL that bananas are sterile" trending once every month. Which is frustrating for veteran users
* Reddit arm chair generals/economists/politicians. The points system rewards trendy content and doesn't necessarily mean quality. Unlike points in Stackoverflow, there is some degree of confidence in the users competency.
* The fact that Reddit is mainly populated with technologically proficient people, who probably have some sort of issue, whether it be mental, psychological. Sure we all suffer from our demons but the amount of stuff I read on there makes me cringe at times.
* The "Reddit Contrarian", waiting to snipe your post with their immense intellect
Sure there are many more that could be extrapolated to any other social network.