I am running SharePoint 2013 RTM on one of my demo servers and want to enable incoming email. The configuration is exactly the same as in SharePoint 2010:
Setup your SMTP server to receive email
Set "Enable sites on this server to receive e-mail" to Yes
Verify firewall settings to allow connections to port 25
However, after checking all this, my incoming email was still not making it into my lists. I can see the incoming email message sitting in the C:\inetpub\mailroot\Drop folder and it is not being processed.
By chance, I decided to set "Enable sites on this server to receive e-mail" to No, save and then back to Yes. Then miraculously, the incoming email is now being processed. I have now seen this exact behaviour several times with my SharePoint 2013 instances.