Many of the younger people who use twitter get their political information from conventional news sources or (more likely) entertainment media and peer-to-peer social media shares. They expected Romney to show up as lobotomized love child of Richie Rich and David Duke. They had always seen Romney (or any Republican who had risen to prominence) filtered through a hostile lens.
It was a shock to see Romney as a calm, very intelligent guy, with plausible reasons for all of his beliefs. It was also unpleasant. They were honest enough to admit that Romney was winning but, the sound of Romney making sense was causing a painful buzzing sound in their heads. Why wasn’t Obama shutting Romney down with a great line (especially one that could reassure the anxious listener that Romney was actually wrong)? Why were the people running the debate letting Romney get away with this?
But the first debate also showed that persuasion is a process rather than an event. Romney gained about four points in the Real Clear Politics polling average, but the mass of people who recognized that Romney won were not going to change their minds – even if they couldn’t explain why Romney was wrong.
They already associated Romney with everything bad.