The rejoicing continued as it was confirmed that Kane was feuding with John Cena as opposed to Mark Henry. A feud with Henry would have almost certainly been a rehash of Henry's ongoing feud with The Big Show, and it's widely accepted that the quality of the matches that the two would have had would have been...lacking, to say the least. Instead we get Kane verse Cena, where speed will play a factor, plus Kane's playing the heel, which has always been given his strongest performances.
Badass Decay: Some will point to the Katie Vick angle as an example of this - and it certainly counts - but most will point to a single moment: when Kane lost his mask. Many fans feel he's never recovered from this.
The moment itself is being treated as Canon Discontinuity by WWE these days. In a recent storyline Kane stated that he was unmasked by The Undertaker (though he was unmasked by the Undertaker at one point a couple years before his permanent unmasking, he put the mask back on afterwards).
Remember, the person who said Undertaker had unmasked Kane was Kane himself, and in kayfabe, Kane's always been a little...unbalanced. So Kane could have convinced himself that it was Undertaker who caused him to permanently unmasked, even though it was actually Triple H who was the mastermind behind the unmasking, because he hates Undertaker that much.
In an interview, Glen Jacobs claimed that the only people 100% behind the unmasking were Vince McMahon...and himself. He felt that they had taken the character as far as they could behind the mask, and that the addition of facial expression allowed him to add more depth to the performance. He also noted that it was becoming increasingly uncomfortable and difficult to wrestle with the mask on, and that removing it was a relief.
Really, aside from the Special Effects Failure, the unmasking itself wasn't a problem. In the months immediately following the unmasking, Kane committed such acts as throwing Rob Van Dam(his friend at the time) through a steel cage, setting Good Old J.R. on fire, tombstoning Linda McMahon, electrotorturing Shane O'Mac's testicles with a car battery, beating Shane in the first ambulance match by tombstoning him on the concrete floor, and burying the Undertaker alive. He sent the whole faction of Evolution running scared from him when he made his entrances. The problems started when they put him int he world title picture for all of a month so he could destroy Goldberg so that Triple H could beat him; after that, they didn't have anything for him to do up until the Royal Rumble where they started teasing the Undertaker's return, and from that point it was only a matter of time until he was used to worf to the returned Deadman. Once that storyline was out of the way, they didn't have any idea what to do with him, and it all went down hill from there.
Crowning Music of Awesome: "Slow Chemical", Kane's entrance theme for years after he'd gotten unmasked (and for a while prior to the unmasking).
His first theme went well with how his character debuted, and his newest theme sounds a bit like his first theme without the guitar riff. Both are awesome.
His latest theme since remasking is an awesome mix of the two which actually fits his persona.
Magnificent Bastard: Revealed himself to be this after using both the feuds and times he teamed up with The Undertaker for over a decade as a way to exploit his weaknesses and then playing the role of the avenging brother in 2010 all the way to the World Heavyweight Championship. (see Xanatos Roulette on the main page)
Special Effects Failure: This has happened to Kane a number of times over the years; sometimes it's been a part of an angle, sometimes it's been a legit malfunction. His unmasking is probably the most notable one, as the supposed 'burns' on his face ended up looking like smeared makeup (which it was). It had to be distorted in later promos to make it look scarier.
This one is even funnier because they eventually DID go off.
This inevitably happens in the Inferno matches. When someone finally gets lit aflame, it never looks convincing. May be a big reason why WWE never really uses the match type.
He was suffering from severe Badass Decay even prior to his unmasking. One needs only refer to any incidents that could be classified as Crowning Moments of either Funny or Heartwarming that occurred while he was masked. For example: the 'Kane-o-roonie'. The friendship with X-Pac. The relationship with Tori. (Both turned on him and lived to tell about it.) His first words being "Suck it" - the D Generation X catchphrase. Tag teaming with the Hurricane and saying "Freaks are cool!" Many feel when he started talking (i.e. acting more and more human), he lost his threat level.
Worth nothing that it was Kane's decision to lose the mask, as he said it was becoming more difficult to wrestle with it on and that he felt limited with the character since he had to convey emotion with just his body. According to reports, only Vince McMahon was fully on-board with the change, but if there's one guy you need to sign off an idea...
Also worth noting is that while even though it was incredibly unpopular even at the time, the unmasking did a lot to undo his Badass Decay; it wasn't until the Royal Rumble the following year where they started building up the return of the Undertaker, who Kane would eventually job to at Wrestlemania XX, that the decay began to set in again. For several months before that, Kane was nothing but an unstoppable monster.
Just when he was becoming a Badass again, the recent storyline in which Paul Bearer is abducted by Edge has managed to decay it all over again, with Kane insisting he's a human being with feelings, and that he loves the holidays and just wants his dad back.
It also just smacked of desperation, since he was beginning to sense Edge was just continuously one step ahead of him.
Also worth mentioning is the time in early 1998 when Paul Bearer had Kane show off his 'powers' by shooting lightning at the stage and setting a camera man on fire with his pyrokinesis.