When a character dies he doesn't just go to "resurrection lobby" and wait there with tea and cookies until the party gets him back into his body. No, he's somewhere where he shouldn't be and from which the burial ceremony should help him journey onward. That means that he is not in a pretty place.
A burial ceremony is meant to take place a short time after the person dies. That means that those days of mourning are tolerable by the soul in whatever place it happens to be. If a ceremony isn't performed in the period stipulated by the character's faith then something nasty is prone to happen. If the character is left without resurrection and without a burial ceremony for a long period of time, then whatever comes back may not be desirable at all. Stephen King's "Pet cemetary" is close to what I'm thinking about.
So here are a set of guidelines for putting this into play:
- The character can be dead between 4 to 7 days without any noticeable effect.
- After a week the strain of being dead and not journeying onward begins to place a toll on the soul.
- 7 to 14 days dead
- Lost memories
- Childish behavior
- Fixed on some though or event
- 15 to 30 days dead minor mental disorder
- Hypochondriac (inventing illnesses)
- Phobia (based on some even early in life) : water, heights, animals, darkness, etc.
- 30 to 60 days dead, major mental disorder
- Neurosis : axiety, panic, cleaness, etc.
- Homicidal rage
- Manic Depressive
- more than 60 days dead (might not be getting your friend back)
- Split personality
- Paraniod Schizophrenic
- Demonic possession
As you can see this puts a bit of pressure on the party to get the character revived as soon as possible. Even when illnesses can be cured by magic too this doesn't relieve the fact that the character will be afflicted by it and he nor the party know exactly what the illness is. This should be played by the GM by a gradual revelation of the illness. If the character develops an phobia to rats it would not be known until the character encounters rats. Greater illnesses may not be even aware to the character. Going on a murder streak may seem the most normal thing and the GM could include it as a series of deaths occurring in town, only later to reveal that the character is the culprit. Illnesses may remain dormant until triggered by some outside event, like stress in a dungeon, combat or serious injury.
In general this should not stop players from reviving their characters. It should put pressure in doing so quickly, not just journey two more weeks in a megadungeon and then get the character revived. It also adds a spin to the returning character, which should not be life ending as it is also curable by magic. You just need to know what illness it is and find the proper cure.