One of the first things that I notice is that you're a relatively new level 80, however you've been diligently running heroics and have yourself in nearly all epic Emblem gear after only a few weeks. Your persistence has paid off, and you're ready to break into the big time. You're very close to being raid-ready, but there's a few tweaks we can apply that will have you at the very top of your game.
First of all, you'll want to continue running the daily random heroic for your free 2 Emblems of Frost every day. You seem to run plenty of heroics, and so you'll have enough Emblems saved up in no time. Make sure to get yourself an invite to complete the weekly raid quest (available near Violet Hold in Dalaran) which will yield an extra 5 Emblems of Frost. Finally, hitting up the Vault of Archavon on a weekly basis will score you a few free set pieces if you're lucky, and Emblems of Frost if you're not. These are relatively easy raids, and a great way to break into the raiding scene. They're easy to PUG, and I find the best time to get a group for these raids is the night after the weekly maintenance. Everyone's lockouts have just reset, and there'll be plenty of people looking for a talented rogue to bring along.
Talents and Glyphs
Speaking of talent, we will definitely want to examine your current build. I see that you're picking up Opportunity and using the Glyph of Garrote, both of which yield a powerful opener. However, this setup is not optimal for pure DPS, and moving those two points from Opportunity to Relentless Strikes will give you a more consistent rotation and should bump your DPS up a few notches. You'd swap out your current Glyph of Garrote for Glyph of Tricks of the Trade, which will boost your tank's threat and/or increase your group's damage, which is a win-win. In addition, I notice that you've picked up Blood Spatter instead of Fleet Footed. Mutilate rogues don't really use Rupture any more, except to keep up Hunger for Blood, and the extra movement speed from Fleet Footed will increase your uptime on mobs, which in turn is a DPS boost. Even though your 2 piece tier 9 bonus buffs Rupture, it's still not worth using. Choosing between these two talents is really a personal call, but Fleet Footed also allows you to use a pure DPS enchant on your boots instead of a run speed enchant.
Enchants and Gems
Now that we're on the topic of gear enhancements, let's take a look at what you've got so far. It looks like you've made the right choice for your meta gem with a Relentless Earthsiege Diamond, and you've got your Nightmare Tear located in a very strategic spot that yields an extra 8 agility. It's all the expertise gems that have me worried, especially the expertise / stamina gem in your shoulders. Once you've met your meta requirements with the Nightmare Tear, you never want to use a blue or half-blue (purple, green) gem again. Ever.
In addition, you are over the expertise cap of 26 expertise, which means that your shoulder gem and one of the gems in your pants are completely wasted! Get these replaced with another red gem ASAP, and enjoy the free DPS stats. Dropping those two gems should have you fairly near the expertise cap, so you won't have to worry about gemming it again. You do have a few pieces of gear missing gems/enchants: fixing those will yield a pure DPS increase.
Your cloak could use the agility or haste enchant, your chest piece would love Powerful Stats applied, your Dirk of the Night Watch needs Berserking on it immediately. You can also add an extra socket to your belt via an Eternal Belt Buckle, you can buff your pants by applying Icescale Leg Armor, and if you pick up Fleet Footed, your boots get some love via Greater Assault. Getting all of your gear enchanted/gemmed is an easy way to boost your DPS.
The only piece of Emblem of Triumph gear remaining for you to buy would be the Mark of Supremacy. The extra hit is fine if it doesn't push you over the White Hit Cap, and it would make an apt replacement for either of your current trinkets. Once you start getting a healthy Emblem of Frost income rolling, there are several immediate upgrades you can take advantage of. The Herkuml War Token is an ideal and amazing trinket for Mutilate, and will be better than anything you can find outside of ICC25 or Death's Verdict.
You'll also want to work on picking up your 2 piece tier 10 bonus quickly, as it's a huge DPS boost and will get you into the habit of using Tricks of the Trade on cooldown. I always recommend buying the shoulders and helm of our tier 10 set, as the shoulders provide some much-needed hit rating, and the helmet is the only set piece that isn't missing a socket compared to a similar ilvl equivalent. In addition, you could always get lucky and have a piece of your tier drop from VoA, so there's no use waste Emblems on a piece you might get to drop. You can also score an amazing belt and cloak from EoFs, which will be the best you can find for some time to come.
If you've got some money in your pocket and don't know where to spend it, you could get a few crafted items. The Bracers of Swift Death are relatively easier to acquire now due to the Frozen Orb changes, and you're a skinner so you can farm most of the materials yourself. You could even make the superior Knightbane Carapace, which would trump your current set piece and give you an extra socket to play around with.
The key is to keep on working on acquiring Emblems of Frost, as these are going to give you access to some amazing gear. You can use any excess Emblems of Triumph to start buying gems to fill out your character screen. I assume you've got your rotation pretty well set; just remember that Deadly Poison goes on the fast dagger, Instant Poison goes on the slow dagger, and that Vanish is actually a DPS cooldown. Your email said you were seeing 4k DPS, which would actually be on the high side for you in a heroic dungeon. Remember that without raid buffs, any rogue will do far less optimal DPS.
Your DPS will jump up by a few thousand every time your party size increases (5-10-25). The trick to topping the heroic charts is liberal use of Tricks of the Trade and Fan of Knives. The best way to top your 10 man charts is to maximize your available buffs by using food and flasks. The key to being #1 in a 25 man raid (and this actually applies to the previous tiers as well) is to maintain as close to 100% uptime on the boss as possible, and to use every cooldown you have as often as possible. Keep up the good work, and let me know how your new few adventures go!
Are you a Rogue looking to up your game? Check back every Wednesday for the latest strategies in Encrypted Text! Get ready for Icecrown Citadel with Ready Check and our guide for Rogues, part 1 and part 2 (Plagueworks), part 3 (Crimson Halls), and part 4 (Frostwing Halls). Starting a new rogue? Check out our leveling guide, starting with creation and levels 1-10, levels 11-50, levels 51-70, and levels 71-80!