I realize that I can not prove this, and people are fine to disagree, but I personally believe that Blaine Gabbert and Mark Sanchez underachieved because they weren't on the bench as a year of on-the-job training.
The draft is about improving your team, and that doesn't always mean improving it immediately. The desire for instantaneous results is why GMs are generally awful at their jobs.
A great shot is when you pull it off. A smart shot is when you don't have the guts to try.