Heyman was a master at accenting his talent's strengths and hiding their weaknesses. Look at Public Enemy : exciting in ECW, rubbish everywhere else. He also knew how to do a long-term build for a match while still keeping the fans interested i.e. Taz vs Sabu. Unfortunately he wasn't the best bookkeeper and had habits of breaking promises, not calling the talent back and having to fly by the seat of his pants because sometimes he didn't know what he was supposed to be doing. Jericho makes him sound scatter-brained. ECW was also a haven for substance abuse and many of the guys that walked through their doors have either needed rehab, died from complications caused by their problems, or killed themselves.
Interestingly, TNA has been going longer than ECW ran for, but I doubt they will be considered to have left the same mark when they wrap up. Which will be in the next 12 months, I feel.