Why do some aircraft require type ratings to fly them? I know it might seem like a silly basic question, clearly some of these aircraft are awfully complex.  I doubt anyone questions the idea of a type rating for a 747-400.  

But where is the line between, "sure, if you can fly you can probably fly this one" and, "you need to know how to fly this plane in particular".  What sorts of functionality cause that sort of distinction?
The FAA specifies the airplanes that require a type rating in 14 CFR 61.31:


  Sec. 61.31 - Type rating requirements, additional training, and
  authorization requirements.
  
  (a) Type ratings required. A person who acts as a pilot in command of
  any of the following aircraft must hold a type rating for that aircraft:
  
  (1) Large aircraft (except lighter-than-air).
  (2) Turbojet-powered airplanes.
  (3) Other aircraft specified by the Administrator through aircraft type certificate procedures.


So in short, large aircraft (meaning that they weigh more than 12,500 lbs.), aircraft having turbojet engines, or any airplane deemed suitable complex that it needs it by the FAA need a type rating in order to fly it as PIC.