I can think of a couple major points that have caused massive culture shifts on reddit (some might say for the worse). There have been other temporary shifts/events, but I think these ones were longer lasting * The "Digg invasion", when Digg 4.0 came out in 2013. This is when reddit turned into mostly memes. Before that, the frontpage was closer to slashdot, but much more open conversation around it. (of course there…
The downvoting thing is interesting.. I fairly often upvote comments with 0 if they haven't said anything bad or objectionable just to make things more fair. The mods on subreddits are a problem too. There is no recourse. If they decide to ban you for any reason, that's the end of it, even if no rule has been broken. If you message modmail to ask for more information, they can just mute you from messaging modmail.
An absolutely site-killing problem. If you actually try to pursue specific topics on subreddits, you find they're administered by a bunch of babies wallowing in their little sandboxes and throwing tantrums for no apparent reason.