I personally don't think there are "restrictions" - just preferable draft areas for certain positions. I'm not saying you cant spend a second on a RB, hell, if the need is RB and there is a sure fire guy there... Go for it. But it's a position that's best suited near the end of the build IMO. Those young guns can really light a fire under a decent offense. Primarily it's a position that doesn't take much transition time. Until the core of the team is built, it's a mid round pick item for me. Mid round QBs primarily don't do jack. You'll find some decent mid round Guards, but rarely LTs. You can find serviceable mid round WRs but very few CBs that can keep up... RB is one of those positions that I think warrants the 3rd-4th round range unless the team is inches away. An elite to great RB can carry a team, but very few push it to the show by themselves... And if you're at the beginning of a build or even 1/3 in, it's hard to spend a top pick on a guy that could be worn down and average by the time the rest the build catches up. It's just strategy to me. It's easier to go to the show and WIN with a youthful RB.