While Adrian Peterson was stellar last season, let's remember that we don't know if he can hold up yet and that even if he does he is the exception to the rule.
How so? Take a look at the RBs drafted in the Top 15 since 1990:
Blair Thomas (MEGA BUST)
Leonard Russell (terrible)
Garrison Hearst (decent career, not worthy of a third overall pick in a deep draft however)
Jerome Bettis (first worthy pick, but let's remember that Pittsburgh saved his career)
Marshall Faulk (definitely a worthy pick)
Ki-Jana Carter (bust by injury but still a bust)
Note: The 1995 draft was especially funny because the first round saw Carter, Rashaan Salaam, James Stewart, Tyrone Wheatley, and Napoleon Kaufman selected while Curtis Martin was taken in the third round and Terrell Davis was taken in the sixth)
Lawrence Phillips (Mega Bust)
Tshminga Biakabutuka (Bust by injury)
Eddie George (the third worthy selection)
Warrick Dunn (never great, but worthy enough)
Curtis Enis (Mega bust by injury)
Fred Taylor (worthy selection)
Edgerrin James (very talented back)
Ricky Williams (Though he had good years, I have to put him in the bust list)
Jamal Lewis (good career)
Thomas Jones (bust)
Ron Dayne (bust)
Ladainian Tomlinson (might go down as the best RB ever)
Ronnie Brown (jury still out)
Cedric Benson (will be a bust)
Cadillac Williams (will be a bust)
I am not going to include Bush, Peterson, or Lynch because they haven't been in the league long enough.
So you have eight worthy picks, two middler guys in Ronnie Brown (right now) and Garrison Hearst and eleven busts. If you include all first round backs, the numbers get even worse.