The thing about Obama v. Romney (as Adam pointed out last night) is that while Obama definitely stumbled and it hurt his poll numbers, he was able to recover in the next two debates because he was the incumbent president. Questions about experience, temperament etc. were irrelevant because for the last four years he'd already proven he was more than capable of doing the job competently (whether you agreed with his politics aside). He could convince voters who'd previously supported him to come back because he had a record.
Think of it like an employee at a firm for the past 3 and a half years who's proven himself to be a dedicated worker and you just catch him on a bad day. For Trump this was his job interview.
Very, very good point.