I used to have an addon in WoW that when I was tanking and someone pulled aggro it would send them a class specific whisper;

"I think you've angered the boss, might I suggest casting Earth-Shield now?"

to a Shaman for example.

"Hey that's great DPS, I think the boss is hungry for some Lazer Chicken now"

for a moonkin druid, etc.

You could go in and edit the whispers too, great fun!