>Okay, he answered top voted questions at the time
Well, for example, he was asked if he'd consider raising the budget for NASA, his answer didn't directly respond or answer the question, just re-quote his campaign piece on the issue. He didn't answer if he'd consider raising the budget, he just gave some purposefully vague reply about the notion that "we should stay at the forefront of space exploration."
>There weren't any different or insightful questions actually asked
More commonly AMAs are deemed "good" if the subject doesn't blatantly just answer the easiest questions asked and goes out of their way, even just a bit, to offer some unique or exclusive insight. Which Obama, or whoever it was, did not do.