I've been seeing a lot of people that assume humans go to the Spirit World when they die. But there's not really any evidence to support this. Iroh was is the only currently known person to continue existing in the Spirit World after his body died. But I think this is an exception to the general rule. He knew what he was doing. He meditated to the Spirit World when his body grew old, and just never meditated out. His body died, but he continues to live on as a spirit. This would explain why he is never seen bending in LOK. Probably because he can't any more.
The only other person whose fate we know after death is the Avatar. Avatar Wan was reincarnated into the next nation in the cycle. Presumably air. And so on. In this sense, Korra is Wan. They are the same person; just in different times, with different circumstances, and in different bodies. But I don't think reincarnation is necessarily a property of Raava. I think all people in the Avatar world are reincarnated when they die. The only reason the Avatar is able to commune with their past lives is because Raava is able to remember.
If everyone went to the Spirit World when they died, there would be millions or billions of souls wandering around. Korra would have bumped into a ton of other people before meeting Iroh.
Another possible explanation is that death is final. There is no afterlife. Or maybe humans are reborn into the Spirit World as spirits themselves. Or maybe there is a different world that they go to altogether. But I like my reincarnation theory the most. What do you guys think?