The most signifcant thing to do is to respec.
Ditch imp disarm, imp shield bash, imp taunt, tactical mastery, imp revenge and improve shield wall. They are all worth skipping in a threat spec. Put those points into cruelty and one hand spec. I'd recommend putting what you have left over into deflection and imp thunderclap.
My shield ranking goes: [item]Aldori Legacy Defender[/item], [item]Dawnforged Defender[/item], [item]Shield of Impenetrable Darkness[/item].
Gear wise, work on getting [item]King's Defender[/item] or the [item]Quickening Blade of the Prince[/item]. Put Executioner on them if you're really concerned about threat. I tend to prefer Mongoose if the weapon will be used all around rather than specifically as a threat weapon. Pick up the [item]Band of Determination[/item] and [item]Nightstrike[/item] from normal Magister's Terrace. Also work on farming the [item]Shard of Contempt[/item] from Heroic Magister's. Get the [item]Brooch of Deftness[/item] from badges.
Enchants&gems: Enchant your shoulder, replace battlemaster with something usefull, try out Deathfrost if you want to be cheap, stick with Mongoose or Executioner if you can afford the mats. If you do want to stick with gemming for socket bonuses, at least replace the parry gems with dodge gems.
You can tank 25mans just fine without most of those talents. Imp Shield Bash, Imp Disarm, Imp Revenge and Imp Taunt are pretty much worthless in 25mans.
You could make a case for Imp Disarm if you're having trouble against Attumen, but if you're doing 25mans you shouldn't need Imp Disarm against him. Everywhere else, it's essentially a worthless talent.
Imp Shield Bash is essentially worthless and is only good for pvp.
Imp Taunt is good if your threat sucks, but if your threat is good you don't need it. It's better to get 2% more crit and hold threat the whole time, than it is to lose threat and taunt it.
Imp Revenge doesn't work on most 25man mobs, and only on a handful of 10man mobs. Stunning a mob may make you live a little longer, or let you interrupt a spell if your shield bash is down, but a stunned mob doesn't hit you, which means you don't get rage, which means you don't build threat.
In short, it's good to want to be a good 25man tank, but those talents aren't helping.