First off, your gear is absolutely fine for any heroic dungeon. I don't know about the latest Icecrown ones, they might be harder but not impossible. Never tried them with less than 5k gs. Talent build is also good. You could move 3 p in Imp HS to Cruelty but
For being a fresh 80 you seem to have geared up pretty quickly. Get the Glyph of Indomitability as your next emblem piece. Go craft the boots, chest, bracers and belt if you can afford it, they will last you all the way to ICC. If you cant afford them, there is a few good pieces in hc dungeons to pick up on the way. Get the Sentinel's Winter Cloak for your first 50 frost badges and – hey – full epic.
Holding threat is harder these days, when dps is handing out crazy numbers. A few suggestions that might help you:
Mark a skull, just any mob so dps can focus on that. Which also means you can focus on that mob and get a good grip on it. If you have aoe heavy dps:ers that wont work ofc.
Vigilance the DK, or any high threat melee dps. I usually put vigilance on melee, because if they do get hit I instantly have taunt fresh. If a caster takes aggro it's easier to see and you can intercept/ intervene.
I try to stay ahead of the group, charging away before last group is all down. So I get a head start on next group. Backside of that is I'm also getting out of range from my healer. I tend to use some of my cooldowns here in the beginning of trash fights.
Make sure you have shockwave ready at the start of the fight as well.
Get the Glyph of Cleaving instead of the Devastate or HS one
Rotation should be something like this:
charge, (heroic throw any caster to silence them and make them come to you,) thunderclap, round them up and shockwave. Start working through the mobs tabbing, devastating and shieldslamming, cleave, cleave, TC whenever it's ready to use. Save SW for next pull.
And then, cheer for loot.