I made some test with friends on Test. And the threat meter is really helping everybody to focus on his job.
The most surprising thing is the fact that aggro tend to move pretty fast at the begining. Numbers will jump back and forth. But the longer the fight lasts, the more stable numbers become and it becomes easier to manage.
During my tests what I found quite usefull was to switch between def and off. I opened with def, launched my 2 taunts, kick, lay waste and taunting blow, basically all CA who have a threat component. Then I switched to off to dps until taunts are back.
It required me to change my hotbars layout to clearly separate threatning CAs from others.
It's a bit annoying to constantly switch, but I hope to get used to it.
... it's not as if we will have much choice anyway.
