Introduced with GU 51, the Threat Meter is was often called the Hate Meter while in testing.
The following is from EQII Players:
The threat meter is an addition to the UI. It will give you a visual representation of how much you're angering the NPC. The tank will be at 100%. After that, whatever percentage your meter shows will let you know how close you are to the tank.
The higher your percentage the more you're annoying the NPC. When you start creeping too close to your tank, it might be time to start using your detaunts! The threat meter is toggleable (Ctrl-T or /togglethreatwindow) so it can be turned off if you don't want to use it.
At this point, you're high on the hate list. This is about the range secondary or off-tanks want to be for peeling purposes, or for quick saves.
You are about to steal aggro. If you don't want to, now is a very good time for your detaunts, or to get up and grab a drink.
This is where you want to live if you don't want to get hit. As a healer, the detaunts drop you very quickly.
Congratulations! You are at the top of the hate list and are the focus of the mob's ire!
Fighters are now loosely grouped into single target and group target tanks. While the idea was to make everyone capable of tanking a heroic instance, different tank classes will be more effective depending on the type of encounter.
It also means that spells have been consolidated and that stances have been enhanced to be more effective. Several stand-alone buffs have been merged into stances in order to support the intended use of that stance. For example, stamina buffs are an aid to tanking and would be merged into a defensive stance.
Guardians, Paladins, and Monks will fall into the single target group. They will be the most effective at engaging a single NPC. In raid situations, this would mean that you'd want one of these guys to keep the main NPC's attention. The same would hold true running through an instance where there is a main NPC who has minions as backup.
Berserkers, Shadowknights, and Bruisers are group target (or AE) tanks. They will be the most effective at scooping up several NPCs and keeping them occupied. They'll be excellent at off-tanking, but by no means does that mean that they can't be the main tank in appropriate situations.
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