99,9% of people doesn't care at all about how technically innovative something is, they only care about how useful it is. And Twitter is incredibly useful.
"Incredibly useful" would imply that if you told me about Twitter, I wouldn't believe you. As much as I wish it were otherwise, it's not that hard to believe that someone invented IRC over HTTP and gave it a silly name.
Many people, myself included, were skeptical of the value of Twitter until they used it. In that sense, we literally did not believe how useful Twitter was. Also, Twitter is almost nothing like IRC in any respect other than being an electronic communication medium.