I do not think that A.J. Styles "sucks" but I do agree that he will never be a big star. Granted, "big" is subjective. I just do not think he is a main event caliber athlete. In this business, especially (maybe even exclusively) in the United States, the most important thing in being a professional wrestler is the ability to talk. That is something that all of the greats, from Ric Flair to Steve Austin, have in common. The Rock could talk. Mick Foley could talk. Hulk Hogan could talk. If you can't talk, the best you can hope for is a Chris Benoit style career where you get a "thank you" run at the top for years of being great in-ring.
You have to be able to make people buy into not only your character but your opponent and the upcoming match as well. You have to make people care. If you have the coolest spots in wrestling, it doesn't matter if no one cares about you or wants to pay to see you win/lose. Styles has no aura about him. He's just a normal looking dude, with a southern accent and occasionally makes you say "ooh" or "aw!".
Granted it is not just a problem with TNA, talking in general is a lost art. Especially with how over-scripted and over-produced wrestling has become. When everyone on your roster has wrestled everyone else on your roster and you still have 12 PPV a year, and a 2-hour TV show a week, people are going to lose interest quickly.
"Boy, can you make folks feel what you feel inside?"