I think its legacy is trying to continue the Attitude Era without most of the major players who made the Attitude Era great and an adult alternative to what wwe was producing. While TNA has people who were a part of the Attitude Era, they really didn't have any of the main guys early on. They had Russo who worked for both companies so he could write that kind of show but the problem early on was they didn't have the top talent to support that. Jarrett was the top guy and while he did work for both companies during the Attitude Era, he was never a top guy or someone people think of from that time. That happened a lot - Raven, Rhino, and others were all part of it but they were not a Steve Austin or Hogan. Eventually they did get some names like Angle and Sting but it seems like even then, there was always that reference back to the Attitude Era but it was too late for most fans as you could go to Raw and see a lot of the guys there instead or they had already been on Raw and kind of did their last run so you weren't seeing their big return to wrestling, you were seeing them after they left wwe.
I do think they will be remembered for putting on a more adult product than wwe. I agree with what people have said - they have done some fantastic matches and I think that has a lot to do with them not catering to the kids. It allows a diversity in the matches because you can use moves that others don't compared to wwe who has banned a ton of moves over the years. I think to that by bringing people from different promotions and letting them be more or less themselves, they created an interesting product compared to wwe where before you hit the mian roster, you get trained to work a wwe style no matter how many years you have wrestled.