The Significant Mistake: Is there anything he actually does right? He can't control Anakin; then he chooses to hide his son on a) Dad's home planet and b) using the same surname; and finally he spends several decades sitting in a cave doing bugger-all. (See also: Yoda.)
 
Impact On The Saga: A completely baffling delay in his doing anything about stopping the Empire.  Frankly, it was good luck that R2D2 landed on exactly the right planet first time, otherwise Kenobi would still be counting grains of sand.
 
If The Force Had Been Strong With Him: He'd have adopted Luke as his own son, and trained him as a Jedi from birth.