It doesn't just end there either Peter. I was about to post something similar to this too, but you beat me to the punch. I wanna preface this thing by saying that I legitimately like Cena. I think his in-ring work is massively underrated, and when he isn't telling corny jokes his mic work can be special. Still, I think one of the main problems is that creative keeps putting him with the hot new commodity, hoping that somehow that'll get him over with those that dislike him. The only problem is, all they really accomplish is getting that hot new thing to completely lose it's steam. Peter you gave some examples of this in your OP, but there are even more.
Let's start off with The Rock. The fact of the matter is, when Rocky first came back there was an extraordinary buzz around the WWE. Everyone was pumped for a while. Cena and Rock were trading verbal blows, and Cena delivered some great material in his promos. Material that almost any other wrestler could have delivered and gotten some of the people on his side, but since it was Cena, all it really did was get a lot of the cynics out there a reason to start hating The Rock.
You mentioned Ryder, so I won't go there into detail, but it's the same type of thing. Ryder was getting over big time. Involve him with Cena, and now most people are sick and tired of Ryder.
How about what is imo probably the worst offence, and that's Big Johnny. John Laurinitus had a pretty damn entertaining storyline with CM Punk going on. Big Johnny was everything that Punk wasn't. Punk was cool, brash, cocky, anti-authoritative, basically everything SCSA was back in the day, and Big Johnny was the slimeball, sneaky, asshole boss that was his rival, just like McMahon was in the attitude era. Their rivalry could have been all time great, but it was cut short because Johnny was picking up steam as a heel, and who else are you gonna give that guy to besides your top gun, right? Nope. What made the Punk/Johnny storyline work was the two polar opposites clashing heads, with the bad guy calling all the shots against that badass anti-authoritative guy that was so over. So they give Laurinitus to John Cena, someone who's always gonna tow the company line, and the storyline falls completely flat. People realized that Big Johnny is just kinda lame. He's bland and boring and can't carry a heel storyline unless there is that polar opposite to go against him. All the sudden, Big Johnny is off of TV and the WWE missed a huge boat with him and Punk.
Peter, you mentioned Ziggler, but how about AJ Lee? She was another one that was super-over. Her hot crazy chick gimmick was the most over thing a Diva has done probably since the Lita and Edge storyline. You have her with Punk and Bryan and Kane and that storyline was pretty damn fun. WWE could have carried that over into the Diva's division where Eve was the top heel, and her and AJ coulda had a feud that people probably would have cared about, something that again hasn't been done in ages. Instead though, all of the sudden you stick her in a business suit and make her the GM, pretty much just to give her an outlet to be stuck with John Cena and eventually turn heel, and now the people turn on AJ.
Now Cena's stuck with Ryback and The Shield, two of the most over commodities in the WWE today, and I just hope creative doesn't bury their future again. Thing is, I would have really liked to see Cena vs Ryback, but the Ryback heel turn just doesn't make sense to me. They coulda both been faces pretty easily in this feud. Why can't Ryback just say he wants the title? What's to stop him from thinking he's just better than Cena and that he deserves it more than him? Why do we have to turn this monster into a whiney little bitch complaining that Cena didn't have his back? It makes Ryback look weak! Compound this with Ryback's first Raw as an antagonist to Cena, where Cena calls him out to the ring at the end of Raw, just for Ryback to come out, stare for a little while, and back off like he's scared of Cena beating him up or something. Why the hell does that make sense for Ryback's character?? It doesn't. It only serves to make Cena look like someone to be feared.
Now, with last night and the Shield, I just don't know. The worst part about all of this is, besides the Big Johnny instance and the Rock instance, all of these examples me and Peter mentioned could very well be the future of this company. One more thing. I'm not blaming Cena for any of this. I don't think Cena's the type of guy who wants to bury people. I don't think his ego is that big. Like I said in this post, he's just a guy who's gonna tow the company line, and I think that's even more disheartening.