Defining the default mail account in Apple's Mail.app for Mountain Lion (10.8) is not intuitively obvious. Where you used to be able to sort the order of accounts within the accounts pane of the preferences, that is no longer possible. This can be particularly annoying if your default account should be your work account, but your personal account is first in the list.
Fortunately, it's not that hard, just different.
If, like me, you keep the list of folders on the left hidden, go ahead and reveal them. Then, expand out the Inbox to show all of your various inboxes.
Now, simply click, and drag any of the inboxes into whatever order you want, making sure that the account you wish to be your default is on top.
From now on, if you do not have a specific message open, or are not looking inside of one specific mail folder, all new messages will be - by default - from the account at the top of that list.
Replies will still be from the account that received the original message, and if you're looking at a folder that is only tied to one account, that account will also be the "from" account when creating a new email.