Jews, and some Christian denominations, believe that writing the "name" of God is a sin. Since God is more or less a title in English, most don't have a problem with it. Depending on what Christian or Jew you ask, God' real name is Jeh_vah or Jahw_h.
Sorry about the underscores. Not taking any chances, don't you know.
Rastafarian's shorten Jahw_h to just Jah for similar reasons.