In OS X 10.8, it is not possible to modify the order of accounts via drag drop, as it was in previous versions, in order to set the default email account. It is possible to set the account from which new messages should be sent in the Composing tab of the Email preferences under "Send new messages from". However, this is not ideal because it overrides the account of the currently selected mailbox. Ideally, the drag/drop behavior would be restored as it was before. This would allow the following:
1. The account of the currently selected mailbox would be used if a specific mailbox is selected.
2. The account of the currently selected email would be used for reply/forward.
3. The default account (based on account order via drag/drop) would be used for new messages if a specific mailbox is not selected (e.g. if you are in the "Inbox").
The above behavior is currently impossible due to drag/drop having been disabled in the account ordering. The best that can be done at the moment is to select an account in the Composing tab, which overrides the currently selected mailbox.