There is no doubt that Joanna was a unique woman. On first glance, she seems like the ideal Survivor candidate: strong, intelligent, a people person, physically capable. However, she is one of only a handful of Survivor players ever to bring to the game a serious dose of heavy-duty religion. This was, ultimately, her downfall.
It's not that there is anything anti-religion about Survivor, but people in general, whether religious or not, don't like people pushing religion down their throats. Joanna did this in a big way. I mean, the Immunity Idol being anti-God??? You are guaranteed to make enemies.
So, although Joanna was voted out as "a threat", a huge chunk of the reasoning behind it came about due to a general desire to bring peace to the camp, minus the tension that Joanna brought in a big way.
It is a shame that she couldn't contain her enthusiasm a bit more, because she was a very interesting woman. I do think she would have had a lot of trouble maintaining any kind of strategy long term. I think it would have been a moral issue, and an issue for her sense of integrity.
Joanna does lose points from me for picking on Christy, and being the bully that we all hate. The tension makes great Tv, but doesn't win fans among the Christy-ites.