Death sucks because ghosts cannot be seen or heard or touched; they are tired but dare not sleep; they are hungry and thirsty but cannot eat or drink; they are always being hunted by the reapers; they cannot leave their remains, which smell terrible to them.
As time goes by, their sight and hearing deteriorate until they are blind and deaf. They hate the living and will try to steal their bodies. Ghosts feed on fear and use it to attack the living, hoping to knock their souls out of their bodies. Ghosts can be placated - a shaman can give a ghost temporary use of his body in exchange for vows of peace.
If you're a bad person, your ghost will smell bad and attract the reapers; if you're a good person, your ghost will smell pleasant and attract angels. If a ghost doesn't reincarnate within a century, he/she/it will simply cease to exist.