One thing that really annoys me from Reddit is the mob mentality.

For instance, a person will make a seemingly "logical" argument and everyone will just keep adding their own "opinion" agreeing with this argument to be part of this mob. Later, even if this argument is proven wrong, you will get insulted, downvoted and threatened if you make any sort of counter-argument. I hate generalizing but my experience was that most people were unwilling to take back what they said.