Many people found that they did not have the mail that was previously stored in their mailbox prior to the reset.
This is, of course, a big deal for bank alts and others who send tens of thousands of gold through the system. Blizzard has shut down the realms again, and is aware of the problem.
Update at 3:35pm: Syndri tells us that they are actively looking for a fix, and that as of yet none has been found. See our realm status post for more information about the extended downtime.
Update at 10:34pm: Mail will be reimbursed. More information.
We here at WoW Insider would like to encourage Blizzard to just fix it!
Stay tuned for more updates.