Brett Gardner's value lies almost exclusively in his feet. His speed allows him to hit triples and doubles, not his bat. He obviously has tremendous plate discipline as evident by his OBP and K/9 or BB/9 rates. He is fairly consistent throughout the season and more importantly, does not have major struggles with RH or LH pitching. He has no power and no power potential, just blazing speed and good on base skills. Oddly enough his OBP>SLUG. That doesn't happen often and whlie OBP is more significant to run production, it isn't as significant without some power potential.
All this contributes to one central idea about his prospect ranking: He has two skills. When his speed goes, and it will, his career will go in the tank. Especially fantasy-wise. As his speed begins to decline he will no longer be able to beat out ground balls, stretch singles into doubles and doubles into triples. As his BA goes down, so will his SLUG and OBP.
For more on this see the career stats of Vince Coleman, Omar Moreno and Delino DeShields.