When Lex is with his mother after he "dies" and she is warning him about all of the pain and destruction he will cause, he is wearing a glove on his right hand. In the comics, Lex wears a glove on his hand which was affected by too much exposure to kryptonite.
This episode shares similarities with the movie "Flatliners", in which med students routinely kill themselves and come back in an attempt to find the answers about what exists beyond death.
The symbol on Ally's door when Chloe visits her is an ankh, the Egyptian symbol of life.
When Johnathon says that Clark is a symbol of justice it is a reference to his future as Superman and the famous tagline "Truth, justice and the American way."
Chloe talks about Lana taking a one-way cruise down the River Styx. The Styx is a river in Greek mythology that formed the boundary between Earth and the Underworld.