TNA's biggest mistake was Hogan and Bischoff. They were good for the company short term, but long term they took their money and ran. They were supposed to be the savior of the company, but they just made it worse and honestly put the final nail in the coffin that is TNA.
Crazy as it sounds, behind the scenes, I think Hogan did some decent things. He really pushed for Austin Aries back then but Dixie put the belt back on Jeff and turned Aries heel which infuriated Hogan and killed the spirit of a lot of fans, myself included.
He brought back the four sides ring which they should have stuck with, Styles has said that Hulk was very engaging with the talent and really tried to help them on their promos.
He also pushed for "Option 3" with the X division title which was brilliant but Dixie ruined it by putting the belt back on Jeff as soon as he got clean.
I thought the presentation under Bischoff-Hervey was much better than now.
Problem with Hulk was too much TV time and horrible creative in the company, but especially his angles. They also paid him performer money when he wasn't able to be a performer anymore.
The guys going behind Hogan's back (and Russo's) and convincing Dixie to change booking against what Hogan and the writers had planned killed the promotion. If you hear some of the past talents talk about some of the original plans they were actually quite decent.
That being said Brooke Hogan involvement was awful, Aces and Eights had good potential but was ruined, and the Claire Lynch storyline was the worst angle in modern wrestling history outside of Kane/HHH/Katie Vick.
I also enjoyed jaded AJ styles, some of the aspects he has kept in his current character, until they hot shotted it and turned him back to generic high flyer too quickly.
Also kept Tenay at PBP too long, most boring PBP guy in history though I do enjoy his color commentary.
Jeff Hardy stumbling to the ring high as a kite for a 30 second main event on a PPV didn't help matters.
Typing all of this, if TNA isn't dead by now I'm not sure what can kill them. Some rough years!