It would be nice to customize messages related to features that are set at the system level such as the Message Size Limit Exceeded message. For example, in the case of this specific message I'd like to be able to add direction to users that try to send oversized messages such as reduce the number of attachments, zip the file(s) or use Filr or the network share to share the file rather than emailing it. I find that when users experience the message they just create a helpdesk ticket and either demand that the limit be removed or ask how they are supposed to send their message otherwise.