Since I was there at the time, I remember. The answer is: one thing, and one thing only. He was a heckuva good TV talk show host over the period of several days right afterwards.
I don't want to minimize this too much. It actually mattered to a degree. A key virtue is that he was actually calm. With all the grief and shock in the air, he helped New Yorkers to feel better.
But that was it. So far as actual emergency management is concerned, the less said the better. Think of the lost command post that he put on top of the WTC, possibly as a love nest. Or the dead firemen who would have survived if they could have communicated by radio with the police. Or the lack of health precautions for people at the site, who are still getting sick due to his negligence. Or the fact that he spent more hours at Yankee games between 9/11 and the end of that year than at the site (not that he was really needed there).
Good talk show host for a few days. Period. Via a completely different persona than his mad dog spewings on the campaign trail these days. And again, this was genuinely valuable and I still appreciate it. But it is a slim reed for a Presidential campaign.