Besides the fact that one can trade half his lifespan for improved usage of the Death Note,
The human who becomes the owner of the Death Note can, in exchange of half his/her remaining life, get the eyeballs of the god of death which will enable him/her to see a human’s name and remaining life span when looking through them.
A user also can write down their own name in the Death Note. This will cause the user to die early due to their own described causes. Of course the rules of the book still apply, meaning that the user's death as written in the note wont take effect if this occurs after his already stated point of death. Therefore, one can shorten his life using the note.