The answer, of course, is just a few small things. It's the little things that separate the great Rogues from the good ones, and it's the little things that will have healers messaging YOU for a Heroic PUG instead of the other way around. Mastering a few of these techniques will help you keep your head above the flood of 70 Rogues who rerolled back when we were the flavor of the month.
I feel that the most important non-DPS skill that a Rogue brings to the table is the technique known as 'peeling'. It refers to the idea of peeling the skin off a banana or other fruit. Similarly, a Rogue can peel enemies off of a party member or arena partner, allowing them to put distance between themselves and the enemy. While we perform best while up in an enemy's face, our caster friends like to have some breathing room. There are many different styles of peeling, and it has applications in both PvE and PvP. Let's first talk about the idea of peeling bosses and mobs in PvE.
While I certainly don't look down on the efforts of Gaeowyn and other Rogues who have perfected the art of tanking raid bosses, I'm talking more about picking up Prince Malchezaar for 15 seconds in Phase 3 after your tank goes down. As emphasized on fights like Reliquary of Souls (phase 1), Rogues can actually tank fairly well using our high passive avoidance and phenomenal ability Evasion. While not suitable for main tank purposes, I have saved several Karazhan PUGs from a brutal wipe by picking up the boss and burning down the last 10% life while I use both my Evasions.
Many Rogues are used to the idea of Vanishing as soon as the tank goes down, and hiding in the corner to avoid durability loss. But if your raid has a boss close to death, consider sticking around and using your 15 seconds (or more) of near-invulnerability to give it one last push. It may make the difference between a long walk back and a pile of purples in the end. And it positions you the closest to the boss when he dies, so you can get the first chance to skin the corpse. Your skin, your hide; right?
On the same topic, I talk to countless PvE Rogues who don't have moves like Blind or Gouge hotkeyed. All players and many mobs are vulnerable to Rogue CC techniques, and they can be used defensively to protect a clothie or to give an incapacitated tank a chance to regain threat. Also, with the new Deadly Brew talent you will be able to keep Crippling Poison on targets as well, reducing your need to Shiv (which used to be the quintessential peeling ability). With Deadly Throw available, we can also peel even targets that are far away from us. As a Rogue, we get CC'd a lot. Learning how to keep your allies safe while also being locked down is an art that only experience can teach.
Save a healer's life (and more importantly, durability) just once and there's a good chance you'll be getting more than just a Renew while eating some nasty Cleaves from the next boss. No healer enjoys playing while they are getting beat up by a thug, so keep them safe and they'll be much more relaxed. I feel that having the situational awareness to know when to stop stabbing and to start CC'ing is a trait that will be appreciated in any context.
A skilled Rogue knows not to wait until the enemy is Mortal Striking your healers to start the peeling process. In my experience, a Rogue's place in every group should be 20 yards ahead from the rest of the party. It puts us in perfect position for a quick Sap, and allows us to scout ahead for possible stealthed targets before they have an opportunity to one-shot our clothies. As a Rogue, if you're in the back, you're slowing your group down. Be ready to attack immediately, don't leave open ground between yourself and the target. The only thing more frustrating than waiting for a caster to drink is waiting for an Energy user to simply walk to the target.
Similarly, in a PvP setting, getting the opener and starting a CC chain is often what decides the pace of a battle. Many arena matches are over as soon as one Rogues get the Sap off on the other. Using Distract to turn enemy Rogues around (and not facing you) can also prove useful when trying to get the first opener. I suggest dueling your friends and experimenting with how close you can be to a target before you're detected, and practicing the proper techniques for approaching a target who knows you're there. If I see a Rogue getting Flared out of stealth more than once, I know he's off my arena team without a good excuse. Knowing the limits of what you can pull off will prevent costly mistakes for the rest of your party.
Using your crowd abilities offensively and with proper team coordination can easily swing the pendulum of a fight from wipe to win, and I can say from experience that it's more important to know your role in a CC chain and how to peel enemies off an ally than to know how to spam Hemo Mutilate.
Let a few healers die; word will get around that you can't peel and you'll never find a Disc Priest for 2v2 again. Save the Mage when he pulls crazy aggro from a Living Bomb crit (trying to pretend that every Mage I see in arena isn't Barrage spec right now); and he'll be calling you for his guild's next Gruul run. Put other players first, and you'll see that they might stop making jokes about your name (you're the one that chose CountGankula) and start inviting you back to that WSG premade.
While not traditionally conceptualized as a defensive class, Rogues possess the capacity to protect our party mates. Not using these abilities to the maximum of their potential will limit your effectiveness, and will have you sitting on the bench while another Rogue takes your spot. So don't let that happen! Peel for your casters, and don't forget to use all your CC options. Your allies will appreciate it.