Forgive me if someone else has written about the draft research from the book Scorecasting by Moskowitz/Wertheim. I thought it fascinating and paraphrase some of it.
The draft value chart which people are familiar with was made by the Cowboys in 1991. It was based on team's values of draft picks, what teams had been willing to trade for over the years. After a while every team had a copy of it. The problem was that it has flaws, notably the over pricing of high picks.
Research showed the following:
"The probability that the first player drafted at a given position is better than the second player drafted at the same position is only 53%, that is, slightly better than a tie.
The probability that the first player drafted at a position is better than the third player drafted at the same position is only 55%.
The probability that the first player drafted at a position is better than the fourth player drafted at the same position is only 56%."
They weren't saying that higher picks weren't better than lower picks, but they weren't THAT much better.
There is also an interesting section about how NFL General Managers are overconfident in their ability to spot talent, as are most humans, and tend to overvalue their instincts when selecting players. Also people value this year's picks 174% more than next years. Quite a nice return on investment for trading into next year. They also point out that picking successful NFL players is really really difficult.
Of course their explanations are more complex than the things I have related. It is an interesting read.