One of the more common mistakes that I see people doing during a fantasy football draft is taking an NFL rookie too damn early. I've seen several drafts where people picked "Run DMC" Darren McFadden of the Oakland Raiders in the early rounds last year because of the hype (as well as Reggie Bush during his rookie year). Although there are a few exceptions to the rule (e.g., Matt Forte from the Chicago Bears and Chris Johnson of the Tennessee Titans in 08), the vast majority of NFL rookies do not fair well in terms of fantasy stats.
The only position that rookies tend to fair well is the running back (RB) position. I managed to snatch Chris Johnson in a trade early on in the year, and he put up some ridiculous numbers for my team. Other positions such as quarterback (QB), wide receiver (WR), and tight end (TE). The only rookie that comes to mind with good numbers was Michael Clayton of the Tampa Bay Buccaneers, who had a wonderful rookie reason and eventually faded into a lesser role in the offense in his next few years with the Bucs. Even last year's best rookie QB Matt Ryan failed to produce any significant fantasy stats for most of the year (with the exception of a few decent games).
So temper your expectations if you do draft a rookie. I would rather spend my pick on a proven veteran instead. The only exception would be if the rookie is a starting RB on a team that does not heavily employ the running-back-by-committee (RBBC) option, and you're not spending a high pick on him.
Thanks for reading,
Mr. F. Nonsense