I disagreed with their false prophesying, unscriptural doctrines, and unchristian practices.
The kicker: I obtained pre-1914 copies of Russell's Studies in the Scriptures. I compared what I found there with the claims regarding that era which the Society published in literature found on the Watchtower Library cd-rom. It became exceedingly clear that the Society's post-1950 claims were untrue. I don't like lies, nor those that promulgate such lies in hundreds of millions of copies over a period of decades. (And who refer to their system of false teachings as "The Truth".)