From my little experience online, the greatest treat/limitation of sites like question and answer is the big man SPAM. And I'm amazed that this has been a top priority here:
1. Report As SPAM, for every Q&A.
Well for my, I think one of the "Must" feature is the ability for user report any question or answer as spam. Maybe a little "report as spam" button after the "comment/reply" buttons will be invaluable. Moderation such site can be a daunting but I think having the user suggest what they think should be moderated will make it work or moderating a lot more easier.
I can see that this has already been suggested before but I just have to reiterate the need. Because it's very "crucial". The importance of "Subcategories" in the organization of a site like this can never be over emphasis. While trying to see the importance of this, I checkout most of the top Q&A sites and they are all making use of subcategories, even check out some other type of site that has to with category organization, they all adopted "subcategories"; eBay, amazon etc.