Health & Finance
In my personal wiki, two of the most important sections to me are my notes on Health and Financial. I organize all my doctor's appointments, for every ailment I have symptoms, diagnosis, treatment, outcome, I have notes about every visit to a healthcare professional, and every prescription. Within finance, I have a page for every account so I have a place to take notes in case I ever need to call their customer service. I can write down when I called, who I spoke to and what the outcome is.
However, because of privacy and regulatory laws, I think it is best if we recommend our users not store any health or financial. Like I said before, I will implement all of the security required to protect health and financial info, but until we have it validated by auditors, best to avoid that at first.