I think anyone that's done some raiding or 5-man end-game instances in Outlands has probably heard of KTM. KLH Threat Meter has been around for probably about a year and a half now. If all members of your group have the mod installed, it works great... sorta. If you've only got one target to kill (think bosses) it's a great way to know that the tank is on the top of the threat meter, and that pesky rogue that does uber DPS is not. It's fairly accurate and has helped a lot of groups with encounters. What did we do before KTM? Who knows?
But what about when we have multiple targets? KTM is an aggregate threat meter. Said in another way, if you, as a DPS'er, switch targets after killing the first (or at any point during the fight), the threat list you see will still be reflective of that first mob. KTM is pretty useless after that first mob. So, how do you know when you'll pull agro on that second, third, or subsequent mob? Bottom line: you don't.
So, while KTM paved the way for threat meters, a new generation is upon us. Enter Omen as the new threat meter. It actually tracks threat on different mobs. So, when you switch from Mob A to Mob B, you can see that the tank hasn't picked up much agro on this mob yet. You don't have to go back to the guess work of old to figure out how much damage you can do before you pull agro.
Sounds too good to be true? Well, there are a few catches. First off, the good folks over at Ace have said they're still in beta for Omen. Updates to the mod happen at least once a day, and sometimes up to 15 times a day. So, some of you might want to wait until release. They've also said that the only thing that's holding them back on final release has to do with pets, but all of the classes (the players themselves) are done. So, they're almost there.
One more word of caution. Because of the way that Blizzard allows you to track different mobs, mobs with the same name and level will share the same agro table as far as Omen is concerned. So, it's still not perfect, but it's a lot closer than KTM is.
Finally, while Omen will obviously work best if everyone in the group has Omen, it will also "listen" to other threat mods, such as KTM, and supply you with that data. As it's stated on the Ace site though, "Please be aware that as KTM is an aggregate meter and not aware of multiple targets, any values from it will be incorrect in any multi-mob encounter beyond the first mob."
I've been testing it on our raids, and it looks really good. Everyone else in our guild (that I'm aware of) is still using KTM, and I haven't pulled agro yet, unless I did it intentionally. So, once it goes live I'll let you know.
One last thing: I've tested a few add-ons recently, so I'm not 100% sure that it's Omen, but I believe that Omen flashes the screen red when you pull agro. If it's not this mod, I'm pretty sure it would be Deadly Boss Mods. Any additional information on this would would be appreciated.