What's the difference with the fans?
When Brock Lesnar got pushed pushed to the top, he faced RVD, Hulk Hogan, The Rock, Undertaker, Big Show, and Kurt Angle. He got the title the same year he came in, and he was beating all of the big names. He beat more big names than John Cena did when he got his major push. He was going to beat Stone Cold Steve Austin as well, but Austin didn't like the idea and the way the WWE was going so he left. He was a heel for half in his run, but he was a dominant heel. Most of the time he was destroying his opponents and he beat all of the ones listed.
When John Cena got pushed to the top, he faced Eddie Guerrero, JBL, Jericho, Angle, Edge and Triple H. He also did it in a slow process in comparison to Brock Lesnar. Cena was also a face. The fans started to boo him when he faced Jericho even when he was a face. When Brock was a heel and when he faced The Rock, they were cheering for Brock Lesnar.
What's the difference? Why the fans turn on Cena, complaining that he was forced down people throats when Brock Lesnar was doing the same thing? Was it because Brock Lesnar was a heel and it was more authentic that Brock Lesnar was a monster that couldn't be stopped? John Cena was a face, who the fans backed up until he faced Jericho.