The biggest probelm I think was that it only had 2 seasons.
The 2nd season's halfway point reveal should have been the finale with an extra third season building up relationships between the twins so when Mabel causes the apocalypse it seems less like being an idiot and more her being hurt that her brother was leaving her behind.
Next was the wheel. First popping up in Season 1, and the crew kept hinting that it would play a significant role because Bill would refer to people by the symbol they represented on the wheel. So for fans waiting months for the finale, only to see it get resolved in a couple of sentences with no payoff, it was disappointing. Like when South Park teased who Cartman's father was and on the day they were supposed to air the reveal episode, they just aired Terrance and Phillip instead.
As the series ended, disappointment became resentment, then anger. Some tossed out the 'Hirsch is a hack' initally as a joke but then people who took it way too seriously picked it up and took it for a ride
Morale being: /co/ always blows things out of proportion. When we say it's the worst thing ever, it's probably because we didn't like a couple of things about it that really bugged us. If we say it's the best thing ever ever, then those things didn't really bug us.