Note that bonuses to saving throws can have surprising effects. For example, a human with the Human Perseverence feat (PHB, heroic tier) who rolls a 19 on a death saving throw can spend a surge to recover, whereas any other character needs a 20 to do the same.
It's not just death saving throws... it's ALL saving throws. That's a pretty good incentive in my opinion.
Rolling 20 on a normal saving throw doesn't do anything special.
Just sayin'.