This might be obvious, but to me, it was an important realization that a lot of subreddits that deal with, say, chronic health issues, are going to skew towards people who have extreme conditions. People who get better aren't necessarily going to want to be hanging around in such spaces.
Even subreddits that don't deal with health conditions are going to skew a certain way over time just as a result of who hangs out there all the time and who is accepted, so the "reality" you see in a subreddit is not necessarily the true reality of the greater population. (Again, this should be obvious, but it's easy to forget this when you start reading a reddit post and thinking, "I don't agree with all these people, but there are many people with this opinion. Should I be thinking more like they do?")
Even subreddits that don't deal with health conditions are going to skew a certain way over time just as a result of who hangs out there all the time and who is accepted, so the "reality" you see in a subreddit is not necessarily the true reality of the greater population. (Again, this should be obvious, but it's easy to forget this when you start reading a reddit post and thinking, "I don't agree with all these people, but there are many people with this opinion. Should I be thinking more like they do?")