I've been on 4chan since 2007 and Reddit since 2012. I'm pretty involved in the Reddit metasphere (Circlebroke and circlejerk) since I can't fucking stand the obnoxious, smug and deluded shithole of a website, and NOT because I am some SJW on a crusade against racism or sexism or political correctness.
For one, I cannot stand Reddits "LOOK AT THIS THING MY _ MADE" show off humblebrag attitude - be it losing weight, doing a thing, playing a game, smoking le weed 420 blaze it etc. Reddit users are NOT as anonymous as they might think and I have made it into a game to peice together clues about redditor's lives based on their endless narcissism.
Second, reddit injects their politics and belief system into every discussion. In the past this was often "brogressive" (legalize weed and gay marriage and tax the rich but keep the icky gays away from us and DAE ron paul?) but nowadays it is more and more in line with /pol/ tier ideology (visit any of the news subreddits for the full experience). I used to think this was /pol/ counterbalancing the obnoxious, smug progressive circlejerk but now I truly believe both 4chan and reddit are infected by the same virus of determined racists and trolls. Reddit's SJWs like SRS needed to be taken down a peg, but not like this, not when having a neutral, rational discussion is already impossible and made worse by shitposting.
Third, reddit by design steals and appropriates culture and elevates it to new levels of shittieness. Reddit "hug of death" has ruined memes and fun things that not even /b/ could fully destroy, and beats a dead horse for an absurdly long time after the initial spark of originality has died out.
If its not the narcissism, contrarianism or content theft that bugs me, its that the sites founders, Admins and mods allowed it to become acceptable conduct and the established norm. >tl;dr If you openly self identify as a redditor, kill yourself, redditors are the worst cancer on the internet.