1. He plays to the younger crowd too much.
So did Hulk Hogan, yet look at how many of us that grew up idolizing the guy still love him. I fail to see why it was ok for Hulk Hogan but not John Cena.
2. Five Moves Of Doom
Every wrestler has a handful of signature moves that they pull out in every match. Randy Savage, Hulk Hogan, Ric Flair, Shawn Michaels, Kurt Angle, The Rock, Stone Cold Steve Austin, The Undertaker, AJ Styles, CM Punk, Rey Mysterio, John Morrison, Daniel Bryan, Shelton Benjamin, Randy Orton, Christian, Sheamus and I could go on. They all had/have their own "Five Moves of Doom" just as the guys who'll be superstars 10 years from now will. A lot of people that just plain hate Cena will hate him no matter what, even if they have to twist a well known commonality like all wrestlers having a handful of similar moves in order to make it fit their own logic.
3. Always has the title.
There are a lot of things wrong with this other very common argument. For one thing, John Cena spent most of 2010 and the first 4 months of 2011 without the WWE Championship. So he doesn't "always have the title". For another thing, John Cena is a legit big draw. He draws in viewers and has for better than half a decade. That means that John Cena makes money. Whether you want to watch him to cheer him or watch him in the hopes of him getting beaten, you watch him. As a result, it's perfectly legitimate that he should be at least within the title picture.
4. Supposedly undeserved face of WWE
As with a lot of arguments against the guy, there's little to no foundation for this common complaint. Cena works his ass off in his matches, delivers promos, is out there in the world promoting WWE 24/7. I mean, it's not like he's gotten busted for drug possession like Rob Van Dam or been named by Sports Illustrated as someone that was on a list of wrestlers said to have recieved steroids like Ken Anderson or is a confirmed drug addict with multiple drug felonies hanging over his head like Jeff Hardy. Can anybody say with a straight face that any of those three is more deserving than John Cena to be the "face" of a pro wrestling company that makes hundreds upon hundreds of millions of dollars a year?
5. He can't wrestle.
If your definition of "wrestle" is flipping and flopping all over the ring, leaping over the top rope in one suicide dive/high spot after another, sacrificing storytelling, basic selling of moves, in-ring psychology all for the purpose of merely squeezing in as many big spots as you can, then Cena can't wrestle.
It might sound like I'm a huge fan of Cena's but I'm really not. I like the guy alright, he does annoy me at times, but the guy does get a lot of unjustified and downright unreasonable hate sometimes.