I really wonder why wikis are so popular. Why would you want to make your readers think YourSpaceBarIsBroken, or that YouHaveParkinson so YourPinkyKeepsHittingThatDamnShiftKeyByAccident?
Yes, I've heard the stories about it making creating content super-easy. But is that really worth making your readers suffer when reading your material? People have enough problems reading properly anyway.
And wouldn't a slightly more restrictive way of creating content make you pay more attention to what you're doing, and so hopefully help with its quality?
On a slightly related note, I ran the advogato main page through the HTML Validation Service, and it came up with the following error, proving that input validation is A Good Thing(tm):
* Line 85, column 52:
It is now <a href=http://www.opencm.org/>released</a> and usable! ^ Error: end tag for element "A" which is not open; try removing the end tag or check for improper nesting of elements