YahooAnswers . . . is perhaps one of the lamest Q&A sites I have found. I think this specifically because when I have looked up things myself I also find someone asking the same question (YES!) and then visit the YahooAnswers site and find that no one has replied and the question is over a year old. Who wants to send a question out into a cyberspace black hole? Not me! How annoying.
I have found some useful content at Mahalo . . . It seems to have a bit more of a developed network than some of the other sites - it is definitely more attractive than AllExperts. (I know, I shouldn't base my opinion on looks but I can't help it when it comes to website content. Sad.)
Anonymity and convenience are the biggest pulls for online reference sources. It's handy to be able to send out a question anytime you have it and check back later when you have time. Also, something someone might do is refer the question to multiple sources to see what kind of quality answers they get so that they can decide which service to use in the future. (I know I've done that with online travel tools to see what's cheapest.)
And...anonymity can be nice when you live in a small town and don't want to ask your local librarian for information on a disease or be seen checking out a book on that topic. Sometimes questions just want to be kept private - not because they are risque in nature but because of the situation.
One of the biggest line learning/reference resources I've ever used frequently is YahooGroups. I've been on one when I was learning how to make handmade lye-based soap. Some things are better learned from someone who's done the work - no matter how good the book reference is it's still a wonderful thing to get information from a pro. Same goes for fixing things. Books and reference desks don't play well into the how to PLUS and WHAT IFs.
I haven't participated in the Slam the Boards event but plant to next month on the 10th.