I dont usually go for cancel buttons on forms but I really think the 'edit post' form should include a cancel button. Some users are not as hugely web savvy as us bunch and part way through writing an edit they may change their mind. Me and you know we can just leave the page but others might expect a cancel button. I think this is pretty needed thing.
I think it might be missing because there are already three buttons and a fourth might be a bit much. Also there's the browser back button or just clicking any other link. Accidentally clicking cancel can also be a problem.
I think Kier or Mike anticipated on having more options, in that case the buttons would not be visible right below the text field which could confuse some users, or forces a user to scroll down even when he/she is not interested in the options below. ATM though, there are not that much options for users, so this does not apply.
I think the reason for having two sets of Save Changes and Preview buttons is the options under the first set. So the first Save Changes and Preview are for the Text Area, and the second Save Changes and Preview are for the option under that if it happened to change anything there so you don't need to go up to press one of the first set buttons. It's the same as it is in vB.
Aaha, I didn't see your post before I post mine, it seems that you posted your post when I was already in the thread.
BTW, at first there was only one set of buttons which is the second, but people wanted to have them right under the Text Area so they were added after.