One thing to keep in mind is the difference between doctrine, personal belief, and tradition. The mainstream churches I've attended or studied make clear distinctions between these things.
Doctrine defines really important matters; the nature of God and Christ, salvation, etc. The church I attended for the longest time had cards set out for newcomers to take home with contact numbers and the beliefs to the church on it. The whole thing was the size of a post card.
Mainstream churches generally allow a lot of lattitude for personal beliefs, and recognize that a lot of things they do are "tradition" which is a big word for "we like to do it this way."
The problem JW's have is that their hierarchial system does not allow any room for anything not handed down from on high. Everybody has to think the same way about everything, whether its the nature of God or when its OK to get up and go to the bathroom. They are also incapable of admitting that they made a mistake, which means that everything has to be "new light." Combine that with an organizational structure that encourages elders to throw their weight around and you have constantly changing beliefs, all of which are strictly enforced.