the few things to help with this are glyphs enchants gems and rotation. the biggest difference i found to help when i ran into this problem was glyphs, horeblast icetouch and dark comand, then i picked up the glove enchant that gives you 2% threat and parry bonus. the main thing about threat as tanking is that you ,untill the expantion, have to have the def cap, but also have to match the caps all the other dps are using ie. hit raiting and expertise so priorty for gems and enchants are def hit expertise parry/dodge stam in that order.
if you find when you get the hang of your rotation and don't have to use dark comand (taunt) as much you should gylph for death and decay instead untill you get the t10 set bonus which will then give you a freeby glyph of your choosing.
the rotation i use with these glyph for frost tanking aoe is:
hb bb bb ob hb bb bb
you should use runic strike when ever it procs throw in HW when you can for a long fight and as you get down to one target switch to BS .also you can start with Death and Decay untill you get the nack of chossing the right target so that Hb infects all targets with frost feverthat would go
DnD hb bb hb bb bb ob
single target rotation would be:
IT PS BS BS OB OB OB OB PS IT BS BS OB
if your team mates pull off you before you can complete a full rotation then there not playing right there either not using tricks, distraction or are just starting in too early. as far as magic users are conserned there will be a problem with crits pulling agro off you untill you get a better weapon/s there is such a thing as a spell hit cap which could benifit you but not a whole lot and i've never found a need all though i have seem tankadins with such a cap. it sucks cause you would have to gem and enchant for some like 476 hit raiting which seems pretty impracticle good luck my toons name is nismo of velen if you would like to se some of the things i'm talking about