Well, he’s a brosonly fan.
He’s probably one of the principle reasons why Misha was fired and Castiel killed off in S7.
He pestered Misha when his wife was in labor for two days and asked him if he could just have the kid surgically removed.
He is married to Eugenie Ross-Leming, who with her writing partner Brad Buckner, writes the worst episodes in the show’s history. And they are clearly only there because Singer is showing her favoritism. They wrote one episode in S1—and then never again during the Kripke years which actually lasted through S6. In fact, Kripke called the episode they wrote “bad writing” on the show (Chuck says the line). Then in S7 when Kripke is gone and Singer I think has bullied Gamble into writing a crappy S7 and consequently getting her fired (or she left in frustration), all of a sudden the Disaster Duo are back. And by S8 they are writing 4 episodes a season, jumping ahead of all the writers that have seniority over them. Why? Because Singer is an executive producer.
I also think he’s responsible for the backpedaling on the Dean/Cas story line in S9 and Ben Edlund’s departure from the show.
He’s also just a narcissistic jerk.
He’s also tacky and classless. He did a “charity” auction at a con once, and while most people do charities that are like help kids and save dogs, he auctioned off two extremely rare items for a lot of money to support what is technically a lobby group—not a charity. It was for a group that is against capital punishment. Now regardless of your feelings on the matter—it is super tacky to bring politics into that kind of situation.
He also Cockles-blocked at SDCC 13.
There are sources for all of these (except the Edlund and Gamble things, those are my own personal conspiracy theories), but I don’t feel like looking them up. Sorry.