It first depends on whether the author has granted permission to others to use, copy, reproduce, process, adapt, modify, publish, transmit, display and distribute such Content in any and all media or distribution methods (now known or later developed). Hosting sites often include such licensing as a condition of using their service. Such a license eliminates any risk of a suit for copyright infringement.
Supposing that there is some reddit-like site that failed to include a such a license for others (for instance, Reddit requires you to license to Redit, but not to other users), an author might decide to sue someone else for copying and distributing their posts without permission. In the worst case scenario, you might end up in court, being sued for copyright infringement. One way to defend yourself against that suit would be to established that is was "fair use", for example if you just copied one sentence, and/or you were making a comment on the text. If the author posted a short novel and you decided to copy it and put it on your web page, that would not be fair use and you would almost certainly be found liable for copyright infringement. Then the question would be, what was the damage? That is hard to judge, but it could be expensive, since you deprived the author of a possibly considerable amount of revenue. The bottom line could be in the millions of dollars, but more probably it would be in the thousands. This article addresses the ways in which it could cost you. It talks about really huge awards up to $221M, or at least a half a million in the case of George Harrison, but literary works are unlikely to be so lucrative (assuming the author is not J.K. Rowling). The cost of litigating the suit is non-trivial: this guy says it can go up to 6 figures. It is improbably that an angry author would bother to litigate a case where you redistribute his Reddit post, since he is unlikely to be able to show much actual damage or loss of revenue and probably did not register his copyright. But if people pay for your pirated works per item, that would be revenue owed to the author, so it could get expensive.