For instance, during his shoot interview with Colt Cabana, he went on about how he had a staph infection on his lower back at waist level that WWE medical missed and stated that by the time the Royal Rumble came around, it'd swollen to the size of a baseball. The only problem was that if Punk had an infection, it wasn't remotely what he claimed it was because WWE put out video of the Royal Rumble showing close ups of Punk's ass & lower back; given that spandex tights are made to fit tight, you would've been able to see a baseball sized lump in the area, but you didn't because it wasn't there. Punk went off on WWE's medical staff and it's why both he & Colt Cabana are being sued by the doctor Punk specifically pointed out, Chris Amann, for defamation. If reports are true, Punk has now severed ties with Colt Cabana because Cabana was recently backstage at a WWE event recently; I don't remember reading why he was there, only that he was there, word got back to him and, reportedly, Punk has ended his friendship with Cabana.
As someone else mentioned, another thing that made me not so sympathetic to Punk was his constant bitching about part timers in WWE using the power of their name to wind up in top level positions on the card. He did the exact same thing when it came to signing with UFC; he used the fact that he had a recognizable name and generated a lot of buzz to be signed to the biggest MMA promotion in the world despite a complete lack of MMA training, experience or fights when there are MMA fighters who try and fail for years to even get UFC to pay attention to them.
Looking back over the whole business with Punk and WWE, his words and actions have given me the perspective that Punk's bitching was ultimately about the same thing others have bitched about over the years: he didn't really give a damn about the boys in the locker room and the only "change" that he wanted was that he was THE top guy in the company. Reportedly, despite being ceaselessly confrontational and difficult, he simply thought WWE should give him exactly what he wanted in exactly the way he wanted it. He stated during the shoot interviews that he wanted the "John Cena push"; that's all well & good but, unlike Cena, Punk never showed any real loyalty to WWE, didn't have Cena's work ethic, constantly complained about everything that was going on as though nothing really made him happy, he was endlessly confrontational towards just about anyone, etc. so it's little wonder why Punk never got the Cena level push that he wanted.
Some do hate on the guy for leaving WWE, or pro wrestling as a whole, just like some still hate on the Rock for going to Hollywood. As I said, I liked Punk as a pro wrestler, but Phil Brooks is a douche bag with a big mouth who doesn't know when to shut up for his own good.
