There are some sites that make it really hard to cancel your sign up to. I've read numerous member's forums where people have requested information on canceling with zero reply.
Sites that deliver 0 content but have a great tour. The biggest example I can think of is
http://hotcollegegirlnextdoor.com, it's definitely the worst site I've ever reviewed. 6 videos in the member's section and if I remember correctly, none of the ones you see in the tour are given.
Silver/Gold or Basic/Advanced memberships some sites have. It's a stupid system and exists purely to overcharge people for their content.
As a reviewer, there are lots of things you find that really disgust you and give a site generally poorer rating. Though one of the most heartbreaking are those that have tours much better than their site content. I really do feel for the guy who doesn't read a review first and gets stuck with a shit site and never wants to buy porn again.