
4chan has a few dozen boards while reddit has thousands upon thousands of subreddits, so you can find a specific subreddit for whatever you want. There's also less choice of boards overall. Reddit users also like the system of upvotes and downvotes because it keeps it so that shitposting is kept to the bottom of the thread. Reddit sees 4chan as being unnecessarily gross (very NSFW, etc). These are mostly just the reasons already stated, but in reverse. Now for the other way around: why reddit hates 4chan. I would argue that it's not really more mature, just more adult, but that's kind of a separate discussion. On 4chan, you don't have to worry about things like that, so it's seen as more "mature". Reddit is also much more commonly used by children and people at work, so there's an entire concept of some threads being NSFW and keeping things (in certain subreddits) appropriate for children. Theists and republicans are usually downvoted to the bottom where nobody can see their opinions. So, because of upvotes and downvotes, the top comments of most threads will favor these viewpoints. Reddit tends to be male, technologically inclined, atheistic, and politically left-leaning. There's also concern about the "hivemind", which absolutely does exist. They also dislike how comments are frequently made just for karma. So 4chan "hates" reddit for being overly politically correct. On reddit, it's much more common to be polite, not insult people, not use racial slurs, etc. There are no accounts, karma, or downvotes, so people say what they really think anonymously. On 4chan, it's common for people to speak their mind without worrying at all about offending anybody. There are advantages and disadvantages to each, but in reality, they're just different.

So most people on 4chan don't really "hate" reddit, but there is still a perceived hate from 4chan to reddit because of some criticisms.

Reddit has a much larger userbase, so it doesn't really go the other way around, however. For the most part, people who use 4chan also use reddit.
