The biggest problems with /busy are A) that it doesn't set or send a return message and B) I don't get an alert that someone has sent me a /tell. /busy would be useful if it worked like this:
  • Set /busy
  • Set option to be alerted (or not)
  • Set return message ("I'm sorry I missed your message -- I'm busy at the moment")
Bonus points if player can design personal return message.

That's the way /away or /afk has worked in pretty much every other MMORPG I've played. It was even in EQ, 10+ years ago. I don't know why FFXIV can't figure out how to implement this feature. Or, if they know how to do it, why haven't they done it?

I made a suggestion about this during Beta...