Mad TV/YMMV

Dude, Not Funny: The Bible Dude sketches.
Hilarious in Hindsight: Their sketch about Ashlee Simpson getting caught lip-synching, done a few years before Simpson really got caught doing so on Saturday Night Live.
Memetic Mutation - Most notably the Can I Have Your Number sketch, featuring Nicole Randall Johnson as a man who hits on women by repeatedly asking for their number.
Nightmare Fuel - Many of this shows attempts at Crossing the Line Twice only managed to cross the line once before falling flat on their face into Dead Baby Comedy. The Curious George parody especially.
- Stuart. When he enters his "dark place". That is all.
They Changed It, Now It Sucks - Much like SNL, a lot of fans have cited many cast and writer changes (and a change in studio and show format in its last couple years) as a reason why the show went downhill, as well as the show being based on MAD Magazine despite that all of the magazine's hallmarks (Alfred E. Neuman and his "What me worry" catchphrase, the Don Martin shorts, and the "Spy vs. Spy" cartoons) were phased out after season 3 and the show became SNL Lite.