Is it their door-to-door incessant preaching?
More often than not, yes. That's the only direct contact most people ever have with the organization.
Is it their view on blood?
For those who know about it, sure. But many don't know about it. I didn't know much about it until I married a JW, and didn't comprehend what it really meant until the Hospital Laision Committee came into the picture.
Is it that the religion itself is simply often misunderstood?
Much of the misunderstanding is deliberate on the part of the WT organization. What they put into writing is much less than what they expect you to do. You are entitled to know next to nothing until you sign up for a study, a little more when you start going to meetings and knocking on doors, but the serious rules only come up after you agree to obey them (sight unseen) by virtue of getting dunked in the pool.