Man, I read a lot of these responses and feel like a manchild, because most of the shows I've quit for this reason have been cartoons. XD
Anyway, for me, there have never been specific points or single episodes that pissed me off to the point of not watching anymore, because I'm a man of great patience and can usually bite the bullet in the hopes that the show in question gets better (I'm usually right). However, there have been a lot of slow burns: shows that gradually started earning my ire until I eventually came to a realization that I wasn't enjoying myself anymore and just gave up.
The big three in my head are:
The Proud Family ("Jeez, some of the characters in this show are assholes")
Ed, Edd n' Eddy ("Jeez, EVERYONE in this show is a HUGE asshole.")
and Foster's Home for Imaginary Friends ("GAWD, Bloo is SUCH an asshole!")
There are probably more, but these are the 3 hugest offenders. I take no shame in saying "screw those shows", I don't care how well received they were.
I also gave up on TTGL, not because of the pilot though - if bad pilots were all that kept me from shows, i never would've bothered with Azumanga Daioh. ZING! Anyway my gripe was mostly during its 2nd season. I get that the show was supposed to be incredibly over-the-top, and I genuinely enjoyed that about it, but at some point after the time skip, it became a game of "who can die in the most explodiest way possible?" That paired with the ending turned me off it for good. Yoko still has a place in my heart though.
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There are a ton of other shows I quit (Spongebob, Scrubs, Family Guy, Naruto, etc), but those were due to boredom rather than anger.