[quote=@pugbutter]
<Snipped quote by Garattee>

Now what is the appeal to [i]Rick and Morty[/i], [i]Steven Universe[/i], [i]Adventure Time[/i], and these other cartoons to which full-grown adults have latched themselves? 
[/quote]

[@pugbutter][@The Elvenqueen]

Hmmmm well. I'll give it a stab. 

I've never really watched Steven Universe, but I'll go with [@Dynamo Frokane]'s answer - as that sounds a good reason to me. 

In terms of adventure time, I've only watched relatively a little but it struck me as a kid's show with lots and lots and [b]lots[/b] of references for older viewers (to the point where I could hardly believe it was a kids show).

I think the joy for most people with this show is the blurred line in target demographic that tickles them. I believe it is targeted at 12-15 kinda age group, however the writers must know it has a huge following with college/university age people too (and older viewers who are still in a similar mindset).  I believe the majority of Reddit browsers you refer to could be said to fit into those age groups. Seems a fairly immature but entertaining show from what I saw. 

Now Rick and Morty I have watched all of, and would have to say I enjoy it lots (as do many of my friends of the same rough age group (late 20s). 

For me the appeal is in the writing and deeper implications of lots of what they cover. It's very dark in many subjects and tickles both the more immature elements of humour as well as my darker streak. It also touches on quite a few deeper things philosophically speaking, if you're such inclined. 

[quote=Wikipedia]
The show touches on an existentialist perspective. Many other Philosophies are referenced like Nihilism and the work of Friedrich Nietzsche, Marxism and Determinism.
[/quote]


Plus I like to smoke plants which helps with the more immature side of things. 

It has had a few great comedians/actors as voice actors too - David Letterman, David Cross, Dana Carvey, Nathan Fielder, Gary Cole, Werner Herzog, Rich Fulcher, Alfred Molina,  Keegan-Michael Key, Matt Walsh, Keith David, [b]John Oliver[/b], Christina Hendricks, [b]Stephen Colbert [/b] & [b]Jemaine Clement[/b]. They've all been in it at one point or another. 

Suppose as well once you get into it (and beyond the slightly less fantastic first few episodes) I really found myself getting to like the characters despite what initially irritated me about each of them. 

And I do not Reddit, so can't blame them this time for the above.

(Edit: Oh, and for those with thick skin and dark humour - veeery dark humour, check out Mr Pickles, for something even more...extreme)