I never thought that this was actually going to be possible back when he first debuted in 2005, but the facts are what they are: Mr. Kennedy was a huge bust in WWE.
His initial foray into the main roster of WWE was phenomenal: his gimmick of announcing himself in a Buffer-esque manner was entertaining as hell, and he had the chops to pull it off. His ring skills, while not phenomenal, were definitely good enough.
His first few months were highlighted by wins over numerous former world champions like Booker T and Rey Mysterio (though with outside interference, of course). He also, on the November 11, 2005 episode of Smackdown, became the last man to ever wrestle Eddie Guerrero, losing by disqualification.
However, he got injured a month later and missed a considerable amount of time, putting a serious damper on any momentum he had. He would mostly get it back after he returned, but suddenly Kennedy's character started to change: he started doing more generic insulting of the fans ("you people") instead of announcing himself and proclaiming how awesome he was.
Why the sudden change? I imagine that, for the most part, it had to do with WWE wanting to keep him heel, and his entertaining shtick that he had developed was starting to get over big with the fans; therefore, he started becoming more generic on the mic.
However, since Vince and many of the other guys in the back were big fans of Kennedy's, his push continued: he was put into feuds with Batista and The Undertaker, including a title shot against Batista at Royal Rumble 2007, and also won the U.S. Title (ultimately his only title in WWE).
The pinnacle of Kennedy's early tenure was winning the Money in the Bank match at Wrestlemania XXIV; however, due to another injury, he had to drop the briefcase to Edge, who would cash it in on a (legitimately) injured Undertaker.
Infamously, Kennedy's triceps injury turned out to have been misdiagnosed and was not nearly as serious as the doctors thought. Had this been known earlier, Kennedy would have likely cashed in his briefcase on Undertaker and won his first world title.
But things didn't work out that way for Kennedy; truly, things stopped working out for Kennedy altogether around this time. It was around this time that Kennedy was suspended for a wellness violation—despite proclaiming that he'd never take steroids and judged people who did—missing out on getting the monster push from the McMahon's Illegitimate Son storyline and a likely world title reign.
Kennedy would get a little momentum back by feuding with Shawn Michaels and then (finally) turning face against William Regal in April 2008, but soon enough it was back to the injured list for Kennedy—who missed almost a year of action.
When he finally returned in May 2009, all the momentum and crowd support he had was gone. Fans sat on their hands when he made his return, and his outstanding promo skills had pretty much evaporated. When he botched a back suplex on Randy Orton on the May 25 edition of Raw, his time was up: He was released a few weeks later.
Mr. Kennedy is a guy that, like Carlito, had all the talent in the world (though Kennedy wasn't as good as Carlito in the ring). However, his constant injuries and inability to keep his mouth shut at the right time sabotaged him in the end. What should have been a WWE career that was highlighted by multiple world title reigns (and one in which he would still be in WWE today) became one of frustratingly unfulfilled promises.
Say what you will, but Mr. Kennedy was a major bust.