The WWF'sAttitude Era gained so much attention because it was so much Darker And Edgier than the days of Superhero-like wrestlers like Hulk Hogan, the fact that much of the new flavor was imported directly from ECW aside.
It could be argued that this was inspired by WCW's taking its two main wrestlers, Hulk Hogan and Sting, and transforming them overnight. The character of Sting morphed from a brightly colored California super-surfer to a grim, vengeful Nineties Anti-Hero, a wrestling version of The Crow. Hulk Hogan became "Hollywood Hogan", an all-powerful gangster with a Hollywood-sized ego. This inspired WWF to change their direction as well as ECW's gaining momentum.
The "steel cage" used in Steel Cage Matches transformed over time from something that kids would probably enjoy climbing at Chuck E. Cheese to, well, an actual steel cage.
Shane Helms' original Superhero gimmick "The Hurricane," seemingly imposed on him in 2001 when he came in from WCW as a joke to amuse the writers, but, Helms' talent, charisma and enthusiasm got the character over with the fans. It helped that Helms actually has a genuine love for comic books. When he revived the character years later just before his departure from WWE, he attempted somewhat to reimagine Hurricane as a grim, silent, Dark Is Not Evil avenger.
Interestingly, while WWE's PG era was initially exactly that, with relatively clean violence and little profanity, it's now gotten more than a bit edgier, the violence is more brutal, the language is dirtier and more frequent, and such Anti Heroes as CM Punk and Randy Orton are starting to become the norm again for faces. Add to that the returns of wrestlers from the Attitude Era like "Stone Cold" Steve Austin and The Rock, and it could very well be only a matter of time before WWE goes back to TV-14. Funnily enough, WWE initially started out as PG before transitioning to the Attitude Era, and then back to PG before starting to get edgier again.
Unlike WWE, TNA had more violence, blood, and profanity and was rated TV-14. It's also been viewed as Incompetence, Inc..
ECW was considered a more darker and edgier professional wrestling organization when compared to other organizations like WWE or WCW back in the day as ECW's storylines take a more mature approach over the cartoony storylines wrestling organizations used to take.
The Undertaker and his Ministry Of Darkness gimmick. Undertaker has always been 'dark' so to speak, but when the Ministry came into existence he become a full blown villain, using creepy religious symbolism, crucifying opponents, abducting women, employing a cult of other wrestlers to do his bidding, the whole nine yards. He became less of a wrestling villain and more of something out of a comic book.
Over the years, Scott Levy turned from the effeminate Scotty Flamingo into the brooding, psychopathic Raven
In his WWE career, CM Punk has continued to become grittier and more visually dark, even when making a Heel-Face Turn
Chris Jericho's creepy video with the children before he returned to compete in the Royal Rumble, lose, and then face CM Punk (then face) at WrestleMania, which had been his plan all along.
With the family friendly + occasional Parental Bonus formula PG-WWE has established with its characters and storylines, the backwoods cult of The Wyatt Family comes off like an Attitude Era gimmick that got off at the wrong stop on a time machine. It has minor shades of the Ministry Of Darkness except that it's a lot earthier and slightly less over-the-top, coming across a lot more like a cult you could very well run across in Real Life if you got lost in the wrong area. Bray, unlike The Undertaker, isn't so much a supernatural being, but he is a man that fully believes himself to be some sort of god, and that arguably makes him even scarier.