In Phillip Jose Farmer's Riverworld series, in the earlier books at any rate, a character who is killed simply wakes up elsewhere, naked, hairless and adult, with full memory of events up until their death. In Roger Zelazny's Lord of Light there are resurrection machines, which will transfer one's consciousness into a new body — though they won't retrieve the consciousness of somebody who's already dead. There was another book I was thinking about a minute ago, but I've forgotten it...
Anyway, as a thought experiment I was wondering how to run a campaign along such lines as a means to handle character death and reintroduction. To maintain campaign continuity, we would have to use the Riverworld-type adult reincarnation; otherwise we'd end up with something more like the Bushido RPG's system of familial development and inheritance — not neccessarily a bad thing, but it does mean that the campaign jumps forward in time every time you want to include a reincarnated character, as you have to wait for them to grow up again.
Note: If the Zelaznyesque reincarnation machines were used, they would have to be modified to work with a corpse to be useful in game terms, and in that case they really just become a replacement for the Reincarnation spell, so I discard them for the purposes of this musing.
We'd need to consider whether auto-reincarnation is universal, or if the PCs are somehow special in that respect. If it is universal, it would have profound effects on laws of inheritance and so forth, and birth rates would have to be rigidly controlled or else the campaign world would overpopulate in no time. Maybe it only works once adulthood is reached?
As far as game mechanics go, we'd need to consider whether the new character appears in its own body or gets a new one, and how much of their memory (i.e. class and level-based abilities) they retain. My personal preference would be to re-roll all character stats except for intelligence and wisdom, and to drop back one, or even two, levels. I feel that there needs to be some mechanical penalty to character death, if only to prevent
Where and when does this new body appear? Maybe there are "resurrection bushes" in whose huge pods grow "empty" bodies; when somebody dies their ka or id (or whatever you want to call it) is immediately transferred to the nearest available fully-developed body. Maybe from time to time un-tenanted bodies also become active.... zombie plague, anyone?
How common are these resurrection bushes? Could every family have one in their back yard, lovingly tended to catch the ka of poor sickly Auntie Flo who's been feeling poorly for months now, or do they grow only in isolated areas steeped in magical mana? If they're common, it would be likely to affect the development of medicine, inasmuch as euthanasia becomes a viable treatment for any serious illness or injury. If they're uncommon, they'd be prized by the rich and powerful.
Is the reincarnation 100% reliable? Or is there a chance of failure? Either option has advantages and disadvantages as far as game play goes. I'd opt for a small, but significant, chance of failure, if only to allow for players who decide that they want to start again with a completely new character conception, and also because I feel that it benefits the game if death is sometimes death. I'd also probably (certainly) restrict it to human characters; immortals like elves clearly have no need for such a mechanism (being immortal), and besides, they get other perks, so screw 'em.
Once revived, the new (old) character would have to find their way back to their group, hopefully to reclaim any of their stuff that hasn't already been nicked by their comrades. They were just keeping it safe, honest.
I think a universal reincarnation mechanic could make for some interesting background details and adventure hooks for a campaign. Handled sensitively I don't think it would upset the balance of most campaigns, except perhaps those dark and gritty ones in which everyone is depressed and doomed all the time anyway. I think it bears more thought.