I think you might be right in that... unique humor is rare enough that it certainly doesn't threaten to overwhelm a site, and it might even represent an increase in overall conversation-intelligence.

Memes, on the other hand, are repeated endlessly. Perhaps still funny, but it's the repetition that drives people away and dumbs down threads, because it contributes literally nothing, but it does distract. It also turns the place into an echo chamber - others were very likely thinking the same thing - and group-think is dangerous.