In Matthew 24:14 - Jesus says "this Good News of the Kingdom". Jesus' words do not allow for a new, improved, or altered message to be preached. In the same chapter, in verses 4 and 5, Jesus warns: "Watch out, and do not let anyone fool you. Many men, claiming to speak for me will come and say, 'I am the Messiah!' and they will fool many people."
Obviously, Jesus did not merely mean that men would always say that they were literally Jesus, the Messiah. It is important to note that Jesus was warning about false teachers that would attempt to attribute Messiah-like characteristics to themselves; namely salvation.
The message that is being preached by the Watchtower organization is indeed a different Gospel, or Good News that was not preached by Jesus or the apostles. The basic thrust of the message is that "honest-hearted ones" must come into "Jehovah's organization" in association with the annointed remnant, or "the Faithful and Discreet Slave". Nowhere in the New Testament is there any indication that people would join a centralized organization for salvation. Jesus in his preaching message never taught that salvation somehow hinged upon association with a seperate class of individuals, or 144,000. He also never indicated that any timetable could be assigned to his return.
The apostle Paul could not have made it more plain when he stated at Galatians 1:6-8:
"I marvel that you are being so quickly removed from the One who called you with Christ's undeserved kindness over to another sort of good news. But it is not another; only there are certain ones who are causing you trouble and wanting to pervert the good news about the Christ. However, even if we or an angel out of heaven were to declare to you as good news something beyond what we declare to you as good news, let him be accursed."