Their elitist attitude that they have a monopoly on the truth and anyone else who claims to be a Christian, no matter how sincere, is full of it.
That, and when I wanted to join the Theocratic School, I had to cut my ties with the Salvation Army because it's a "false religion." I volunteered during a youth summer camp.
Later, I find out that several sisters buy clothes from the Salvation Army. Hmmm, I thought we weren't supposed to be involved with "false religion." I actually voiced this complain to the man who studied with me and even to an elder. They had a few arguements, but had to admit I had a valid point.