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omen is threat meter and its kinda a must for all dps and tanks. http://wow.curse.com/downloads/wow-addons/details/omen-threat-meter.aspx

gemming looks fine.

stacking armour and stamina is the way to go :)

if you worry about threat then no amount of expertise is going to correct that. if you are using your abilities properly is the biggest boost to your threat!.

http://www.tankspot.com/showthread.php?65700-Expertise-Hit-and-your-Threat.-What-it-means-to-all-Tanks.-(Current-for-3.3.3)

dont wory about the loss of expertise :)