Cult - and a very powerful one at that.
It's taken me seven L-O-N-G years to realize this, and I don't make this judgement lightly.
Individually, the cult members are by and large lovely people. However, the rottenness comes out by examining the indivuals as a group... for it is the group you asked about.
The power of groups comes from the alignment of purpose and ideals by it's members. Just look at Alcholics Anonymous. This is a very healthy group.
However in a cult (any cult), the members go beyond aligning their ideals, but also aligning their very way of thinking with each other. This is unhealthy and does not encourage members to actively question the authority or reasoning of the group. In fact, this one aspect alone causes so many other problems it's incredible.
Cults almost always have a "revealed truth" or central person or body that is worshipped. To question this truth is usually punished in some manner. The JW's use shunning... a very common and long practiced technique by many cults.
Cults almost always have a "Us vs. Them" mentality.
Example:
They are the enemy. Satan controls them.
We understand you, We will take care of you, We will protect you.
Cults almost always have "cult-speak"... and the JW's are very well practiced in this. They can instantly judge another member's sincerity to the group just by listening to the buzz-words that make up their "cult-speak".
Example:
"He's a worldly person."
"Christendom will come to an end"... and so forth.
Groups members can (and should) hold common ideals... even if they have different ideas. You may disagree with me, but that only means we share different ideas... not ideals!