Basically, rebirthing is a suicide-like thing where you start anew (doable after reaching a certain total worth), forsaking your experience points, items and so on, while
getting the benefits of a bunch of new features after the rebirth. (new races and such).
So... feel free to discuss your personal opinions here, who knows, we might even end up with something that will be coded

Okay then, let's give it a kickstart, my opinion/idea on rebirthing:
Things concerning the rebirther:
Players keep their masteries. (Masteries are the reward for devoting one's time to a particular guild, should not be erased)
Players keep their marked eq (There should be a wizard to check if the player has given some of his/her eq to another player for safekeeping, so that they won't be destroyed because of the rebirth). Unique and ungettable eq are also kept.
Players keep their permanent divine favours.
Players keep their aliases, nicknames, notebooks and such.
Players keep their houses and furniture (but not the eq in the racks/chests)
Quests that are done are kept. No second time rewards for quests that are supposed to reward the player only once.
After rebirthing:
Players won't get a personal exprate bonus
Players might get access to new rebirth-only races (someone has to code them, though)
Some extra (skills that are previously not learnable by doing) or all skills would be learnable by doing to a certain point (like 30%)
That's all I could think of... Perhaps you have (possibly even better) ideas regarding this topic?