Mr Light wrote:
Doesn't this raise a huge issue, though? If a human just decides to go into the Spirit World they become immortal?
On the contrary -- we were clearly shown the corpse of Professor Zei, who died in Wan Shi Tong's library. If you go physically into the Spirit World, you're as mortal as you are in the material world. Iroh said he "left his body behind." His spirit crossed over through meditation, the way Korra's and Jinora's did, and he left his physical body -- which must've been quite old by that point -- to die. So Iroh is dead, not immortal. But his spirit lives on. Presumably if his spirit had still been in his body when he died, his spirit would have been reincarnated in another body. But by making it into the Spirit World, he's apparently left the reincarnation cycle behind, i.e. attained nirvana.