1. Lack of ability in the pick and roll. From what I've seen, which is admittedly limited, he's not a great distributor which could cause problems moving forward with Favors. This could be mitigated to some extent by the ability of Hayward to handle the ball, but a true pick and roll point guard could help Favors reach his potential.
2. Lack of sample size / high price relative to sample size. Bledsoe has been impressive, but he has played the majority of his minutes off the bench and even though he did start a few games, I don't think he's been scouted to the same extent that most starters have. If he is made a higher priority in our offense, will we find ourselves with a Rondo-like offensive situation, with opposing PGs backing WAY off him and forcing him to shoot? This could be a problem. That, combined with the potential for a serious overpay right now because of the hype surrounding him and the hesitance of the Clippers to trade him could make this acquisition disastrous in the worst case scenario.
I think those are both pretty reasonable arguments against Bledsoe, although personally I am a fan and would be more than happy to trade for him.