The drop-off from year 1 to year 2 can largely be attributed to defenses having had a year to figure out how to stop him. His first year he had some huge games simply because defenses weren't prepared for a rookie to come in and dominate like he did.

Year 2 to year 3 is a little misleading because he got a new quarterback and a new offense, resulting in his rushing numbers being diluted a bit. Note that if you add in his 10.1 yard receiving average, his yards per touch are actually up from last year.

I don't think this is necessarily something to put on Peterson. Over the years, the Vikings have added more offensive weapons, notably a better receiving core and aquarterback. With Brett Favre there this year, the Vikings had a passing threat that was lacking with Tavaris Jackson, so Peterson wasn't relied on as much to run the offense (This is similar in San Diego with LT - his role is reduced now that Philip Rivers has become a better qb). I also think that defenses are more prepared for him than in previous years, when he was first breaking out on the scene.

I think a two-dimensional attack should, if anything, increase his efficiency since defenses can't cheat and load the box with eight guys every down.