It was questions like these that caused the Nicene Council to convene in the first place. That early council asked the question, "What is Christian? and, if those core principles are not followed, they can no longer be called "Christian", or orthodox.
Many, many different churches who quibble on minor matters all subscribe to the Nicene Creed, including both the Catholic and Protestant branches. And, on minor matters, churches change their stance many times. The United Church, for instance, has modified it's definition of marriage to accommodate homosexual unions.
Now, if these churches claimed to be inspired of God as the JW's have, that is the only organization speaking for God in these days, they would be in trouble.
Also, the JW's do not have a similar core set of beliefs that they ascribe to. A Jehovah's Witness in good standing must believe the "current light" of the society, in all it's detail, or no longer be a Jehovah's Witness. This is why we can say the only core belief that the WTBTS holds as sacred is that the Faithful and Discreet Slave must be obeyed.