I've been toying with the following idea:
Toughness grants 3 HP upon taking the feat.
Thereafter, when rolling for HP at each level, the character can re-roll 1's.
If this is deemed too effective, I may limit it by saying that once you've prevented 3 1's, you don't have any more rerolls left. Or similar.
I like the original version. +3 hp is pretty strong for lower levels. I'd "fix" it by making it a prereq for Improved Toughness, then add a Superior Toughness that adds +10 hit points, +1 hit point per level.
That's ... interesting.Toughness gives you +3 hitpoints, and increases your death threshhold by 3 hitpoints. So you die at -13 instead of -10. Also added the 3 hps to anything like Diehard that normally functions until -10.
While I am sure someone would find a way to abuse it that's not even what I had in mind.Honestly I don't remember it ever coming up...
Can't see a reason why you couldnt take it more than once. Unless there is some new ability or class I dont know about that would abuse it.