Long ago when I started playing WoW, one of the first addons I downloaded from this site was named
AggroDisplay. That addon has worked flawlessly up until 4.01 hit.
I am not a programmer, though I do understand enough to see how things work, and what this addon does is very simple. If PLAYER_REGEN_ENABLED is true (essentially meaning that the player is not in combat and is regening health and mana at a normal rate), then the addon just displays an * on the screen. However, if you happen to tick something off and PLAYER_REGEN_DISABLED is true, then the message "Aggro!" is shown on screen. Nothing more, nothing less, short and simple.
Could somebody please take a look at it, the lua is only 25 lines long, and see if you can get it working for 4.01? I would really appreciate it.