The word Christian come from the Greek word for "Follower of Christ". Therefore, there is no such things as a "Christian religion". You are either a follower of Jesus or you are not.
Prior to the term "Christian" Jesus' followers were referred as people who followed "The Way". "The Way" referred to the fact that it was a way of life. This means followin Jesus teachings is similiar to the way Buddhists follow Buddha's teachings.
Jehovah's Witnesses are not Christians because, in order for that statement to be true, they would all have to be following Jesus' teachings. If just one person who identifies him or herself as a Witness and lives a life not in harmony with Jesus the entire group cannot be rightfully called Christian. Jesus never taught that his teachings were a named religion.
Here are some additional questions to ask ourselves:
1) How many Kingdom Halls did Jesus build and teach from?
2) What new publications did Jesus produce for his followers to use in teaching?
3) What name did Jesus give his followers to seperate themselves from those who did not follow him?