I haven't had a chance to go through all of the different threads posted here, but here's one I'd like to toss in for discussion. Please excuse me if it's already been discussed, and if it has, please direct me to that forum. Thanks!
The Gift of Iluvatar to Men, or Mortality, or Death, or The Seeking Beyond, has always interested me since I first read about it in the Silmarillion. I had trouble understanding it at first, but as I read and reread the books, I started to understand, both the concept itself and the conflict it created between Elves and Men. Even now, 30 years after I first started reading JRRT, I'm still groping with the concept of the reason for it.
Eru made the Elves immortal, His Elder Children, to the delight of the Valar. They would be together through the Ages of the World after their Awakening, and not die. But Men would be mortal, and live for a relatively short time compared to the Elves, and then die and leave the Circles of the World. The Elves and the Valar don't understand the reason for this, and neither do Men. The Elves don't "die" even if they are killed: their spirits go to the Halls of Mandos in Aman, and after a time they can be reborn: Feanor is still in the Halls, Finrod was reborn in Valinor, and Luthien was offered the chance but turned it down. But the spirits of Men go to separate Halls in Mandos and then leave the World forever: Beren lingered there until Luthien came.
I honestly think that Iluvatar made it easy for Men when he made them mortal: They get tired of life, since their bodies aren't designed to last longer than about a hundred years or so. The Numenoreans were given a longer lifespan, but again their bodies weren't designed to last longer than that. Men were designed for "planned obsolescence" of a sort, while the Elves and the Valar were part of the Essence and Life of Arda itself. Yes, the Elves and the Valar live forever, but it even says in the Silmarillion that eventually the Valar themselves will get tired of living forever and want to die.
Personally, I would rather stay mortal myself. Yes, the world is a wonderful place despite everything that's wrong with it, but eventually my body will wear out, and I believe that there is a life of a sort after death. Call it Heaven, or Hell, or Limbo, or the Afterlife. Even Earendil was weary of the world but chose to be immortal for Elwing his wife. And at the end of the Third Age, Elrond grew weary of Middle-Earth and departed into the West. King Elessar himself willingly gave up his life at the end and died peacefully, and was afterward joined by Arwen.
What choice would you make, if you could, and why?

Poof!!!! Okay, your wish has been granted, but please try to make your investrigation on your own time and we won't expect a report on your findings

I think that the Gift can't be easily given or taken away, so Morgoth corrupted it. But now that I think about it, it seems to me that we're given an example of what can happen if Men try to defy their nature, or if the Valar or Maiar try to defy Eru's Will in this matter.
For this example, I give you the Ringwraiths.
As Gandalf explains to Frodo, the Great Rings don't convey more life to their bearers. The Men who wear them merely continue to exist until every moment is torture. Or as BIlbo said to Gandalf: "I feel thin, sort of stretched, like butter spread over too much bread." The Ringwraiths became enslaved to the Rings they wore, but I think it can also be said that they became trapped in this world by the Rings, unable to leave. They finally died when the One Ring was destroyed, while the Witch King was killed by Eowyn and Merry because he was supposed to. (We can talk about Doom in another thread.) I like to think that, even as evil as they may have been, they were happy and actually relieved whey they were finally released from their slavery.