Dont forget Flair, Benoit and to an extent, Booker TI don't think WWE ever recovered from the losses of wrestlers such as Austin, The Rock, Chris Jericho, Brock Lesnar, Kurt Angle and Eddie Guerrero (amongst others) - huge personalities and capable of evoking huge amounts of heat or sympathy with ease. Jericho, Angle and to a lesser extent Eddie were also very capable of putting-over new talent - how many new careers has Jericho helped start within the WWE?
I think this is the issue Cena has - he's got to the point where he cannot really lose any small matches without things like interference, whilst retaining a persona that is hugely annoying.
two of the most entertaining feuds for me was Benoit v Angle and Austin v Booker T. The fight in the supermarket was hilarious. "Price check for a jackass!"