Modularity is the word of the day IMHO, to that end here was my suggestion (cribbed from earlier postings).
Give certain spells/abilities keywords, like Death, Petrification, etc, those keywords would dictate things like how they interact with certain defenses or how they are recovered from.
Next based on the style of play you want, you can opt to go with Saving Throw Module A, in which it's one save between you and whatever ill effect you're trying avoid. Or you go with Saving Throw Module B, in which powers and spells that have certain keywords, such as Death/Petrification/etc allow three saving throws, the first fail is dazed, second is stunned, third is the full effect (or some three cascading effects, they should at least be efficacious enough so that using this module wouldn't render such powers not worth using in place of other powers). Lethality could be adjusted by lengthening or shortening the track. Now depending on the average chance of a successful saving throw some penalty after first failed save may be necessary to keep the effect having some teeth, but that sort of thing is a bit hard to say without knowing exactly what the final saving throw mechanic and the numbers surrounding it are.
There are other ways you could further tweak something like the Saving Throw Module B above. You could make magical effects, poison effect, et c. dispellable by an appropriate spell or power if they are used before the duration is complete (incidentally I think this is a more interesting way of using dispel than to get rid of buffs). In the case of magic perhaps killing the caster before the effect takes hold dispels it as well, that would have some interesting tactical implications for combat. A successful save could end the effect, or three saves no matter what so the power is like to have some effect even if it's not ultimately lethal, you could have successful saves move you one step up the track rather than ending the effect until they either get back to step zero or succumb. There are a lot of variations you could do.
For effects that are more about sudden trauma, like a knife in the dark, you could turn to the death by massive damage rule in 3e for inspiration. Since HP don't actually represent physically damage we can tweak this mechanic to mean something else, or a few different things really. You could give run of the mill grunts lower thresholds and more elite creatures like dragons (size) and arch-wizards (wards) and cranky anti-paladins (sheer skill and orneriness) higher ones. You could give them different thresholds based on whether they are aware or not or even other circumstances (maybe a regular threshold and a disadvantaged one).
You could then do things like give Thieves an out of combat damage bonus, Assassins an out of combat bonus and maybe a circumstantial in combat bonus (assuming an assassin is basically a rogue that has given up some of the exploration/social ability for more combat ability), or maybe the ability to combo his smaller damage attacks to count as one for breaching the threshold (assuming multiple attacks are possible). Maybe fighters can attack the lower threshold in combat rather out and vice versa for classes with a Death/Sneak attack.
Upshot of all of this would be that it would make the sneaking into a camp and quietly shanking the sentries a possibility without having to make them low level creatures. It would allow everyone the chance to be able to do it, even wizards, but classes that are better at dealing damage (fighter/barbarians/et c.) would be better at it, and classes designed for such things (thieves/rogues/assassins) would be better still, but on the flip side once the melee starts the combat classes would be able to shine more then it came to reaping through all those grunts and cannon fodder and such.
PC's, being awesome like they are, simply get better thresholds. Thresholds would be a dial that can be adjusted up or down for deadliness.
