Harper essentially played on 1.5 knees this season and still had a 133 OPS+ and improved across the board from his rookie year. At age 20 he already has a very solid walk rate, solid strikeout rates (especially considering his power) and an advanced hitting approach. The things that he needs to work on offensively - not trying to do too much and keeping his emotions from hurting his approach - are coachable, easily.
There is no reason that he can't be a perennial MVP-caliber player very soon, if not this year. Watching this kid play baseball is awesome.