I allow reincarnation as is. It's a cool spell, one of the few in the PHB that isn't generica.
No true rez, but I do allow raise dead, with a minor caveat:
Lord Death is the being tasked with ferrying souls to their various destinations after they cast off the mortal coil.
Once a soul reaches it's final destination, it is either in torment, or in eternal bliss, for the most part. Either way, it is not available for return to the living world.
It is possible to barter with Lord Death for the soul in question. Understandably, Lord Death is not an easy person to negotiate with. She doesn't exactly have many needs in a material way. However she does crave souls, so from time to time she will allow soul to be exchanged for soul. Enter the side quest. Lord Death is Neutral. Good and Evil are not her concern, only souls are. Thankfully (for the pc's at least) those who would cheat death are usually evil anyways, eliminating those pesky alignment issues, but the rat bastard GM has been known to send the party Paladin off on the morally ambiguous quest in order to raise that fallen ally.