Reddit's greatest strength (the subreddit architecture) is also its greatest weakness. Subs attract groups of likeminded individuals usually with a particular interest, and as such a default tone and set of opinions are preset within a sub before you arrive.

If you fail to understand what the preset rules of engagement and opinions are you're going to have a bad time. Worse still, if you happen to disagree with the general consensus of a sub your opinion will simply be downvoted and your voice will not be heard. You're very unlikely to be educated by existing sub members as to why your opinion is wrong with any real sense of perspective because the hive mind has kicked in and you're not aligned with it.

The larger the sub, the more this problem is exacerbated as there are more people ready and waiting to hit the downvote button. This means that you're very unlikely to encounter a balanced and sensible (aka civil) discussion on Reddit.