Other than 1998 when he had 57 points, he was basically at best a 40 point defenseman - mostly in the 30s - and he didn't round out his defensive game early on either. Then he explodes in the 2003 playoffs, wins the Norris in 2004, wins the World Cup in 2004, and had two more elite seasons after that and another Cup. We just all seemed to forget about his foibles before that. Then after 2007 he didn't do anything noteworthy. We are basically talking about a defenseman who has 4-5 elite seasons at the most. Yes, he frustrated you to no end. He had 0 goals and 6 assists during the Devils march to the Cup final. Niedermayer could do that to you all the time. There was always the underlying feeling that he didn't use his speed or ever reach his potential. I still don't think he did.
Think of it this way. Look at the disapointment Jay Bouwmeester has been. Excellent skater, 6'4", good size, long stride. But hey, everyone knows that but him. He's been awful in his career compared to what he "could" have done. In a way, that was Niedermayer pre-2003. If you are the best skater on the ice at all times you should showcase it, you know, like Coffey or Orr did. So yeah, he's in the HHOF someday because he did a lot of good things, won a lot, etc. and it overshadows the "what if" discussion about him.