Nothing enormous; that's basically what PBKDF1 does.  cperciva might correct me on this, but IIRC there are some minor concerns, addressed by PBKDF2, which is a slightly more complicated construction, about the iterated hash function degenerating into cycles.  I don't think there have ever been any practical results exploiting this.  PBKDF2 is just as good a choice as bcrypt, and a better-supported one.