One behavior that seems to span all sports: the less fans know about a player, the more they seem to like him.
The most obvious example of the fervor fans have for a back-up QB when the starter is struggling. If the back-up did anything in a pre-season game, the fans clamor for him when the starter has problems.
But you see it everywhere. Bench players, young players and draft picks -- all are touted above their level of production by fans, until they've actually proven what they're worth. You see it in soccer, basketball, football, baseball.
I've come to the conclusion that when a player is a blank slate, fans are able to project all their hopes, their aspirations onto the guy. As long as he stays a blank slate, that behavior continues. Sometimes it's harmless fun. But sometimes it's really annoying, because fans become so insistent that they're right, without any evidence to support that belief.
Anyway, long way of saying that everyone's an optimist after the draft.