Once a subreddit hits some magic number, it devolves into a stream of image links to "hey I made a cake/painting/object with [subject of subreddit] on it".
Mods can very easily guide users toward discussion. r/atheism infamously banned images altogether, and aside from the "REVOLT I WANT MEMES IN MY R/ATHEISM" posts, people actually started talking again. r/twoxchromosomes has a no-images-unless-it's-Friday rule.
Which is fine, but you have to make some attempt and consolidating that type of content as much as possible. An example being to let /r/gaming run wild with the image macros and meme posts, but then rule those out on the subreddits for specific games. If you don't, you just end up with each game's subreddit becoming basically a filtered version of /r/gaming.
But there are some subreddits that used to be good conversation hubs, that simply got overrun with trash because the moderators weren't paying attention, or were too scared to set up and enforce rules.