Aside from the trolling and otherwise immature behaviour we see in certain online communities (usually ones with a lot of very young people), I've actually observed that people in real life seem so much more complacent and uncritical compared to people online.
Maybe there's a sample bias because of the online communities I visit (like HN) and the real world that I live in, but in general, I've observed that people in real life seem overly concerned about keeping things "harmonious", with all the small-talk, the lack of real listening, talking past each other, not voicing differing perspectives, etc. They also seem to lack patience in various things, whether that's discussing or examining something (can't think of examples right now), and would rather "go back to their own lives". In contrast people online seem so much more generous.
I'm guessing this is partially because "being on the internet" naturally filters people. But what else is there? What's your experience with people online vs in real life? Why do you think this is the case?
In real life, only a few people hear what you say but they feel socially obligated to react in some way. So you get bland, polite reactions. They're the people that, if they'd seen your post on social media, wouldn't have replied at all.
In a nutshell: on social media, you broadcast to a huge potential audience, and your actual audience chooses you. In real life, your ideas only get to a small number of people who may not have chosen to listen to you. The interest levels of these two audiences are naturally very different.