He has made some bonehead throwing decisions at catcher that have hurt them at key moments, but boneheaded plays can be fixed a lot easier than bad fielding fundamentals. To his credit, Napoli's bad plays were in trying to make great plays, but sometimes, it's best to get the easy out or don't try and throw a guy out at 2nd when their's a guy at 3rd and there's 2 outs and K'Rod is dealing anyway, that sort of thing.
I think it's just rookie enthusiasm more than anything. When you blow a game making a bad throw you shouldn't have made to begin with, that sort of thing tends to not be forgotten and you get a little more conservative next time and let K-Rod win the game instead of you.
But boy he sure can hit. What I like is, (1) the look on his face when he's up to bat. He's got a "come on meat, throw it" look to him, and (2) He hits it where it's pitched and isn't trying to go yard on pitches he can't.