Show remaining characters/number of messages as the user types
A little counter in the bottom showing the number of characters typed and the number available would be nice. See Twitter, probably others for examples. Google Voice used to do it but the new UI does not appear to. Note that this will need some minor tie in with #8 - if it's an MMS, the character cap is higher.
Character limit should be configurable, and if it's a hard limit the admin should be able to disable it or make it a soft limit.