Although you all should also remember that Traven willingly put himself into a soul gem. However, it still may be torment but in his case, honor and the destruction of Mannimarco seems to override the fear of endless torment.
~TK.R
Most people here tend to ignore that fact and consider Traven to be an abomination and traitor to the guild sadly.
I personally think you're completely dead by the time you're in a soul-gem
And that's factual. Soul can't sense so yes, sensory deprivation, but more than that. It's fate worse than death, by a mile. Your soul is being torn apart and re-made into raw energies. You are being erased from existence. Now you understand the sacrifice.
Arkay didn't just set his rules for fun, there's a lot of reason why soul-trapping a conscious intelligent being should be forbidden. I would say soul trap of anything is a crime. Imagine if souls were a proven fact in our world, and that they would encompass the very "essence" of being. And imagine someone would be harvesting them for some "cola-loca" drink or whatever. Even animal souls would probably be out of the question.
Of course, once the energy is used, the essence possibly returns, as noted for Daedra. But from what I know there's no knowledge of whether the souls of Daedra get re-made after being soul-trapped and then "used". (I'm sure they can be kept this way, which would be a good war strategy
)