I like ur gear setup. ur talent are ok but i would say drop tact mastery for something a bit more useful maybe imp Tclap for more multi mob tanking or something.
Couple things i would change with ur gear though. instead of the +15 block rating use the 18 stam to shield. Your block% is very nice and your health is a tad low. in that regard maybe throw some staright stam gems in certain places like ur chest to help that along.
also. hit is a tad low only at 15 try bringin it up to 80ish. doesn't look like u r into hyjal/bt yet but it wil help alot with threat gain later on. ur expertise is great though.
Very nice gear setup, I would prolly elect to replace to T4 gloves with T5, and depending on how much excess badges you have, grabbing the phalanx bracer, and the new chest. Also farming for the new Commendation trinket would help the stam out a bit, since you have very nice avoidance already, and could afford to lose a trinket slot.
pepe for general use and threat, slikk's for pure mitigation/avoidance. Swap as necessary if you have both available. I'd just stick with pepe's unless you had extra badges to spend. Gem's are okay I suppose but I'd personally just stack the stam in most places, and a +8/10 hit gem in the gun. Enchant the mallet with executioner for the best threat weapon in the game for a human.
for the most part yes, the expertise that the mallet gives makes it a huge threat weapon. and to reiterate what Satrina says. You have 17 slots to gear for survivability, but only 1 to gear for pure threat. Your weapon is the best place for this (aside from tanking gear that HAS threat attached to it aka Pepe's). +hit? anything up until 142 hit rating will benefit you, so the more the merrier.