I liked him when he was rapper Cena in 2003 because his promos were funny. After he won his first WWE title and then for about 5 months he feuded with Jericho and Angle. Thats when I started to hate him. Cena's matches with them really exposed him. And Jericho and Angle are two of the best wrestlers ever, and even they couldnt make Cena look good.
The thing is Stone Cold and the Rock arent very good wrestlers either but unlike Cena they could still tell a good story in the ring. Plus they were both amazing on the mic.
The many reasons I hate Cena:
1. Cant wrestle
2. Mic skills are mediocre
3. Cant sell moves. See Wrestlemania 23 for the best example
4. Cant act.
5. The FU/AA is a terrible finisher
6. Has the STF as a finisher even though he's not a submission wrestler.
7. Has been face for nearly 9 straight years.
8. SuperCena gimmick where he never loses cleanly, and will get dominated in matches but still wins with his 5 moves of doom.
9. Has been in the main event of nearly every PPV for 7 straight years.
I'm kind of with you in this boat, Other than the whole cant't wrestle and such because people are going to argue with you on that being that people here are sooooo superior and believe that they know what a good wrestler is. I say being a good wrestler or rather an opinion on one being a good wrestler is subjective. It depends on what one likes in a wrestler. Hell, there's people that belive the Divas are good wrestlers.....

But alas it doesnt matter.
Like what you said, he was entertianing as Hell in his rapper phase, especially when he was heel. I mean, he was Cool and thats honestly what I like for the most part. Austin/Rock was Cool. Randy orton was cool as Legend Killer and somewhat as the Heel dangerous viper. I mean these characters made you want to cheer for them because they weren't so bland and voring. In my opinion Cena is boring. He has been for a long time. Ever since the whole Superhero brigade and comepletely turning his back on his old street yet comical like style. Now he's just cheesy, boring and Super's up when he needs too. It's like The Character in the video game that you only play when you want to when because you know he will be invincible, but he has NO style, NO delivery, Nothing to make you like him but the fact that you know picking him will garauntee victory, kind of like Eyedol in Killer Instinct or M. Bison in Street Fighter II. In my opinion theyre both lame characters but they'll OWN you just like that.
I don't really care if Cena goes Heel or not. I just want something to make me go "Wow" or something truly interesting. Punk had it with his imfamous shoot. Even in his SES days he was badass. Now he rarely drops Pipe bombs and he's a boring WWE Stooge. Don't get me wrong I still wach him, hoping he will surprise me but again with the current condition of the WWE everyone is made to be boring.
Back in the Attitude Era, even the era that followed and some of the characters even till 2009 wasnt too bad. The Attitiude Era with Austin who for the most part was an asskicker and fuckss his boss off so the gimmick never got boring. Also because he wasnt a title Hog. He went for the title alot but he had good opponents and other men holding the title that gave good storylines as well as matches with him. Not skilled matches but good because the characters were good. Which includes
Rocky: Trashtalker with a deep heritage who always had to have the finer things in life.
Undertaker: Scary as Hell motherfucker who'd own your ass and take your soul or make you join his ministry.
Mankind: Weird Bastard who'd lure you into Boiler rooms and beat your ass but also kind hearted and gullible.
Kane: FROM HELL. Will destroy you in seconds and though his movelist practually imitated Takers ihe had an awesome costume and mask which made him mysterious and all sorts of pyrotechnics
HHH: Badass yet caniving fuck with a bulked up Chick named Chyna by his side ready to own you and always hungry for more.
Thats just to name a few. SO if Ausitn ever got remotely boring one of these guys would get him in a n interesting storyling, not to mention the Mcmahon family running wild.
Then of course the Later years had Jericho, Angle, Booker T, Test, Goldberg, Rhyno, DDP, Benoit, Guerrero, Edge, CHsritain, Dudleys and so on. So I guess my pint was during those days there were alot more intersting charcaters and developing, which is rare in these days. So in all honesty, Cena's character is lame and washed up because of:
One. Being unable to go edgy hardcore because of the Make a Wish stuff and charities and kids looking up to him and blah blah blah
Two. No one interesting to fued with. After fueding with franchise players Rock and Brock....really, who to fued with next? I say maybe Bryan or Orton but make it good and Cena cant ALWAYS win or be some sissy pushover like he was during most of the Rock fued.
Three. The current state of the WWE trying to blur the lines between kiddy and adult. They don't seem to have a solid foot in either and they're trying to please all, whcih affects the fued and characters.
Four. The internet, dirtsheets and the fact that we all read them and by that we can safely predict what;s comeing next so there's no need to try to surprise us cus if they do what we expect we'll be bored and if they pull a fast one, we'll probaly say we knew it or even if we're surprised later on they'll fuck it up like they did with Brocks return
Five. Vince and the WWE can't keep a good thing going. Everything dies down and NOTHING keeps fire. They had a great season pre wrestlemania and a little after Mania and now its back to boring boring boring. As with Orton in 2009 after he kiced Vince's head off everything was going great and we were waiting to see what would happen next then Game comes back and buries him to Hell...boring.
Six. Cena's merchandise sells. Since he does so well with seels and makes the money so much company with practically everything that involves him because he's moons above all the other talent, due to WWE constantly pushing him. He can't really change to much or do anything drastic cus Vince loves the money and doesnt see a need to chance it.
Seven. Even if Cena does more moves we will still hate him cus he's Cena...he can have the movest of Dean Maleanko but the point is he's still Cena. He still is that name chanracter...Time for a change, a drastic one but like number six...Vince is like ha! Yeah right
Eight. As I said about the current state of the WWE,there are really no division. Just Cena and the few Main Event Players, the Midcard, The jobbers and the Divas...simple as that..Boring. WWE needs to rebuild the tag division and make the champs credible. Same with the "Divas" division. Same with the midcard. The U.S title should mean more. The IC title should mean more, but something entirely different. Jobbers will always be around...then enhacne the main even. Push all of the mian even players and kind of pull Cena back. I mean, they don't have to bury him but just give him less screen time and make him have other goals and such besides the title or whatever the "Mian" thing is.
Now I'm sure lots are going to disagree with me and you all have the right but I did explain myself well and these are my opinions. Shalom