I've never actually been a JW myself, I joined this site to learn about them and once I learned, to keep my wife from joining them. So pardon me if I get some details wrong, but this is what offends me or bothers me most about them:
1) They print their own version of the bible, with alterations that back up their teachings. It is impossible to discuss tenets of the JWs and disprove them from a biblical standpoint because of this, in arguements I've had with my wife, she would just say "but it's in the bible" To which I'd say "yeah, but it's THEIR bible". She thinks they just took out all the thees and thous to make it easier on folks to understand, she does not get that they made key changes.
2) The "opposer" doctrine. I dunno what they actually call this, but it's what I call the practice of teaching people that if your spouse/family members don't "see the light" then they're the enemy. Which sets up "unbelieving mates" and "unequally yoked before the lord" and all that other horsecrap that makes being the spouse of a JW a nightmare. Luckily for me, mine never got that deeply into it, but the horror stories I've read from some people on here that had it far worse than me make you want to strangle the ones responsible.
2b) The "porneia" (sp?) I list this as 2b rather than a seperate point, because I almost could believe that they did this on purpose...it certainly meshes well with the "opposer" doctrine and creates more friction in the home, a sort of synergistic effect, whether it was intended that way or not. Suddenly, not only is the spouse the enemy, but you can deny them certain sex acts and they have no right to be upset about it- and if they are upset, it PROVES they're just an evil opposer sent by demons to "stumble" you.
3) This one isn't even a belief, it's just what is-not sure whether to more properly label it as sad, ironic, or insulting, but the fact that they're a publishing company at heart, rather than a religion. It was so weird when I first learned that there were people called "publishers" within the JWs, I was like...huh? Why a publisher? Then I began to learn how it was all about the tracts and other literature.