Agreed about the speed comment. That's the troubling bit.
I still would take a flier on him myself, but he's no longer going to be ahead of Andrus/Escobar in my rankings.
I wonder how much Kevin is either projecting differently from others or not projecting at all in terms of his power: BA expects Castro to gain speed and power as he fills out and ages. I hadn't heard the speed deal before too much (beyond Canseco claiming that his leg strength gave him speed in the 80s), but that might be the difference between the industry hype on Castro and Kevin's sorta wet blanket (He was very careful in his answers to point out that he still considers him a fine prospect).
An underrated downfall of Castro is the prescence of Hu (I think, I'm sorry, but I have a block on his name). The Cubs have a lot of M-INF talent, and someone who can play SS will get moved off. It's not implausible for Castro to arrive in the show second and get moved off. Sure, G-Becks isn't the defender that Castro is, but he was moved off because of Ramirez, not because the team saw that he couldn't do it.