There's a fairly steep learning curve for defensive linemen. Many don't have the size they need to survive as more than a situational player and the tackles they'll face on Sundays are athletic enough that a single pass rush move won't be as effective as it was in college. So it takes a couple of seasons before NFL strength training and the development of a variety of pass rush moves kicks in.
As JOBOO's data suggests, it's the rarest of talents that contributes immediately.
If you're drafting someone looking for immediate help, you'll need a guy who's got a mostly maxed out frame, an array of moves and opportunity. Only Derrick Morgan and Brandon Graham likely fit that bill this year, and Graham's opportunity should be considered questionable. Morgan is the only player I'd consider draftable in a redraft league, but Graham (and maybe Jared Odrick) are worth watching in camp.
If your roster is deep enough to consider 2010 a wash season, the list of players with the body type, developmental potential and future opportunity expands to include Carlos Dunlap, Corey Wootton and Jerry Hughes. Most would consider Jason Pierre-Paul in that tier, but I'm not convinced we'll ever see 40 solos from him unless he's well into double digit sacks.