One of the concepts that I have found resonating with me from the 2d6 fantasy game is the idea of the experience die. The experience die is kind of like a hit die, except that you spend it to cancel damage when you get hit rather than roll it at the beginning to see how many hit points you get. In my first conception, an experience die could also be used as a sort of wild card fate point that you could add to any other roll (though note that the outcome of this use is not certain in the way that the use of a fate point is). For now I would just like to consider the idea of deferring hit dice.
What if we were to bolt this concept onto D&D? How would that work? Characters would have no hit points, they would just have hit dice. Every time a character took damage, they would spend some hit dice to cancel out that damage, continuing to spend the dice until either all the damage was cancelled or all the hit dice were exhausted. All hit dice being exhausted is much like being reduced to zero or less HP. The character would be automatically knocked unconscious at that point, and would need to roll a save versus death to test for survival.
This does make hit dice slightly less valuable in a mathematical sense, as "extra" HP per die (compared to a given hit) is wasted. For example, if a character is hit for 2 damage, experience dice must be spent to avoid rolling a saving throw. Assume the character is first level (and thus has only one experience die). If the player rolls a 2 or higher, all the damage is cancelled. If, say, a 5 is rolled though, the die is still fully spent, and the next hit taken will automatically cause either unconsciousness or death (depending on the saving throw result). Perhaps allowing experience dice to not be consumed if a six is rolled would be an interesting variant.
What do you think? Hate it or love it? I confess that I am rather taken with the idea, as it allows characters to benefit from advancement while maintaining the tension of taking damage right up to the point of final resolution (a character with six hit dice could potentially roll six ones and be mortally threatened by 1d6 damage, if the damage die came up 6).
Edit: how would healing work? Maybe by restoring spent hit dice. So cure light wounds would restore one hit die.