Sorry if anyone's already suggested this, but I've thought that it would be very helpful if a player had a "..." over their head while they're typing (like on Facebook), or if a player did some motion that mimicked writing or something along that line. It would help let others know if someone is actually typing, or if they are AFK.
Also, as was suggested by Addisond, it would be nice if people that were in your chat window (those that fall under the category of guild chat or personal message) had something beside their names to suggest that they were typing.
And as was suggested by Soulstaker, there should be a way to turn it off if you find it annoying.
Finally, it would be amazing if there could be a pop- up "reply button" by people's names in the chat window that would appear if you hovered your mouse over their name. This would save the time of having to type /t Name or /g and such.
PS: This is going to sound kind of weird, but I had a dream about this last night. I dreamed that when I hovered my mouse over a knight, an ellipses appeared in an action bubble over their head.
Very useful