If his ideas sucked, he wouldn't have had a good 3, 4 years of acclaimed stories before Sins Past came along.
To be honest, JMS' run was never all that amazing. His characterization of Peter, which was really good, was what made a lot of his stories appealing.
The Totem arc never really went anywhere other than vague mystical mumbo jumbo. People say the ending left things open as to whether he was mystical or science based or not, but it really didn't.
The Doc Ock story was alright, the HYDRA story was overrated imo. The Molten Man 2 arc was formulaic.
The Other and Sins Past were complete disasters. He likes to blame Joe Q for making him change the parents of the children but that still doesn't excuse the crap we got.
The only story that he really did completely right was Aunt May discovering Peter was Spider-Man. That was flawless.
I appreciate him getting Pete and MJ back together but the actual reasons behind it were a bit weak. Ah well, beggars can't be choosers I suppose.
The majority of the villains he came up with were boring. Morlun's only famous for his fight scenes, as a character he's got 0 depth. Digger was okay but nothing special, he could have used a few different powers besides just being really strong. For the most part he created a gallery of forgettable rogues.
JMS' Civil War arc left me really cold. Not only did I not like Peter's role in it, he wrote Stark and Reed as total fascists instead of trying to help us understand their positions.