Some of the "skill" positions have lost a lot of their value in the NFL draft simply because of the way players move all over the NFL in the era of the salary cap and free agency. Nowadays, it's just as easy for a GM to fill a spot on his roster through free agency as it is to draft a player. At the QB position, for example, there is a huge risk in using a high draft pick for a college player who isn't a "can't-miss" prospect. You're better off finding someone who has a decent track record on another NFL team and signing him as a free agent.
For a position like running back, there's no reason to draft a superstar if you don't have the offensive line to block for him. In the NFL you're better off with a C+ player behind a solid offensive line than an A+ running back with no line in front of him.
Offensive linemen are highly prized in the NFL draft because with free agency there's no way to spend time developing these players -- so it's important to get them playing together as a unit right from the start of their careers. Defensive linemen are highly prized because the NFL has gone so far towards practically "legislating" record-breaking passing statistics into the game that a strong pass rush is one of the few effective tools a team has to defend against a strong passing attack.
-PJ