I feel a little bad bro, I see where you are coming from.
Firstly something is wrong, a change or something is messing up rotations if you spam. It's like a queue or just plain server lag, but it caused abilities you spam to override abilities you use last second like shield slam on a sword and board proc. It makes everything seem less fluid and in my case incredibly frustrating. In this respect we need to just relax, spam a little less devestate and use our abilities carefully despite our urge to hit buttons like a mad man for max threat. Hopefully they will fix this soon.
In terms of aoe threat, I see where you are coming from and yes simple 5 mans will be a little tricky, luckily most ppl outgear them so it doesn't matter a whole lot. Our aoe threat was indeed nerfed, t clap does much less damage and it is all in the intention that in cata we wont be pulling large packs of mobs and aoeing them down. They want single target and strategy, while you may feel lost now in cata I assure you, you will do perfectly fine. Now for current dungeons I would suggest setting a skull mark to a hotkey that way you can quickly mark a skull as soon as you pull, shield slam that target, use your shockwave and t clap and make sure revenge is always used right away. Personally I would drop gag order for thunderstruck, even though you wont be spamming thunderclap in raids it will still give you a 10% shockwave damage increase and in dungeons, which you are having trouble in, rougly 30% shockwave damage and more t clap damage.
Do not feel bad that you can't make everything stick to you, just do these simple things, be ready to taunt anything that gets pulled off you and when you identify who is pulling the most vig him for instant taunts. If you mark a skull you will probably hold aggro however since it shouldn't really be aoe that pulls off you but instead split dps it should help a ton and it will literally only take a half a sec if you hotkey it to something (mine is set to ~).
I would make enchant and gem suggestions but none of them will help threat, I don't even use armsman atm however after 4.0.1 im not sure how much an armor kit is better survivability wise. I would remove the hit gem from the ring, you can always use hit food if needed. It's being suggested to use hodir shoulder enchant again thanks to def = stam on it. cloak enchant should probably be armor but again idk if anything changed with 4.0.1.
All that is beside the point however, don't be discouraged. They did make us better raid boss tanks and worse at aoe/dungeons, but in all likelihood we will receive some changes soon and/or the instances will change to be in our favor. I am sure it's not you but the situation, this is evident merely from your effort to fix it and embarrassment, which I assure you is fully unjustified.