Actually i saw overhead text in the game once, and liked it, but when i logged back in the next day it was gone. And it showed the overhead text no matter if i talked public or local. Dunno what that was tho, but i took a screenshot. :> The overhead text would be a good idea except for that little bug. To separete local and public i guess you could just color one of them.
The local mute seems useful, there's too many annoying localspammers out there. About the "local messages 15 seconds" idea, i think 15 seconds is a VERY long time, especially if you're used to irc, but if you speak like 5 lines in 6 seconds you could be disconnected, because that's allmost allways a flood. To avoid spamming there could be a block against repeating your message more then 2 times? This would mean them having to type a new word all the time and they would get tired of that pretty quick.