While you've been picking up all of the new ranks of our rogue abilities, they've largely been simple "now does more damage"-type upgrades. We get to see a truly new feature added to an ability once we hit level 61 and receive Garrote (Rank 7). This version of Garrote includes an incredibly useful 3 second silence on top of the normal bleed effect. Try it on the next caster mob you attack, and you'll be pleasantly surprised to see them try to melee you with their feeble weapons. Laugh as you then smash their cloth-wearing faces in.
Immediately after receiving our revived opener, you'll pick up Envenom at level 62. It's a Mutilate raider's finisher of choice, but only if you're using Instant and Deadly Poisons, which not every rogue will have due to more attractive talents while leveling. It also requires you to get a 5 stack of Deadly Poison to deal maximum damage, and so you likely won't have time to use it while leveling on regular mobs. It does Nature damage and therefore pieces through armor, so don't knock its usefulness on highly armored targets. I just didn't use it much at all while leveling, and unless you're looking for a cool new finisher, stick to Eviscerate.
Deadly Throw is our next new finisher, received at level 64. It gives rogues their only ranged presence, and can be useful for taking out fleeing targets. DT used to be far more potent when it had the ability to interrupt via a pair of PvP gloves, but is now fairly limp outside of very specific situations (usually PvP). There are a lot of great macros that can combine Throw and Deadly Throw into one easy-to-push button. Be sure to keep rolling on guns and bows and stealing them from those dirty hunters, because DT is hardly useful enough to warrant restricting yourself to a Thrown weapon. The other PvP ability added, Cloak of Shadows, is far more useful.
Cloak of Shadows will remove nearly any magical debuff from you instantly once you receive it at level 66, as well as providing significant spell resistance for a few seconds afterwards. A magical debuff could include poisons, magic effects, diseases, and other similar ailments. It won't remove physical debuffs like bleeds or some rogue crowd-control moves such as Blind or Kidney Shot. A lifesaver against any caster you may fight, and a short enough CD that you can use it regularly when fighting a magic-using mob. Learning when to Cloak will become a big differentiator between a good rogue and a great rogue, especially in PvP, so bind it to an easy to access key and use it often.
Our level 70 class ability is Shiv, which used to see quite a bit of use in PvP and PvE alike. It was valuable for guaranteeing a poison proc in PvP, but that's been all but eradicated due to the PPM change for poisons and Deadly Brew. Blizzard has also made it a point to completely cripple the ability in PvE, by adding several penalties and nerfs to the ability over its lifetime to ensure it's never a viable option for long. I use Shiv incredibly rarely, almost exclusively on bosses or adds with an Enrage effect that I need to quickly purge via Anesthetic Poison. Keep it on your bar somewhere, but don't worry about having it quick at hand.
Now that we have access to our coveted 41 and 51 point talents, we can finally start the process of building the perfect rogue playstyle. I would suggest looking at two things: what are your best weapons, and what type of rogue abilities do you prefer? If you've got a great pair of daggers, Assassination will be a great way to level up. Once you pick up Mutilate, you're truly capable of dealing tons of damage, provided you have the energy to do so. You also get several useful poison effects, your CDs provide pretty significant burst damage when necessary, and you'll be killing targets before they even have a change to react. With Fleet Footed, you'll be running around much faster than any other rogue. Hunger for Blood will greatly boost your DPS as well, though many Assassination rogues skip it due to the bleed requirement for use. Make sure to only pull one mob at a time, as you have no AoE at all and you have the worst defenses of any rogue spec.
A typical Mutilate build will pick up all of the core DPS talents while also grabbing the quality of life improvements that are the most useful while leveling. Your typical fight will start with a quick opener (Cheap Shot or Ambush) and immediately start using Mutilate. Your poisons will be dealing massive damage and adding several debuffs to your target. A quick Improved (and glyphed) Eviscerate later, and the mob will be at your feet. Quick bursts via unloading all of your energy will be the name of the game. You can use Cold Blood to add some burst against a strong target, and always remember to Stealth between mobs to take advantage of Overkill's extra energy regeneration.
Do you have better combat-style weapons, with a nice slow main hand? Combat provides very constant sustained damage, while having the cooldowns to tackle an elite or two while possessing the strongest multiple target presence via Blade Flurry and Unfair Advantage. If you enjoy a brawler or swashbuckling playstyle, find yourself a slow weapon and spec combat. You have the highest defensive capabilities of any tree via the Stamina, Parry, and Dodge bonuses from talents. You're hard to kill and will be able to easily dispatch any opponent. The downside is that Stealth becomes almost useless for you, and you'll end up just spamming Sinister Strike a lot. Even your 41 point talent is simply a DPS boost (though a huge one). At level 60 you receive Killing Spree, which adds even more potency to your AoE damage and provides another amazing CD ability.
I bet you couldn't guess, but Combat's playstyle is still pretty flat. You'll be using Sinister Strike a lot, while Eviscerating when at 5 combo points. Your white damage becomes incredibly potent via all of the Combat talents, and your critical strikes will be dealing massive damage once you reach Prey on the Weak. You won't have the quick burst of Mutilate (well, without using a CD) but you'll be able to take on mobs incredibly quick, take far less damage, and dispatch multiple targets via Blade Flurry with ease. I suggest using your cooldowns like Adrenaline Rush as often as possible, to maximize their benefit. If you're always "saving" your CDs, you're not getting the use out of them you could be. A rogue has plenty of CDs, feel free to use them all! I simply save Vanish for a bad situation and use my other CDs as soon as the situation warrants it.
Got slow weapons, but the combat way of life isn't for you? Or do you have daggers but just want to try something different? Subtlety may be for you. It possesses very strong opening burst and can give you a lot of control via Hemorrhage's low energy cost. It's also a great option on a PvP server where you may need to defend yourself from the opposite faction: Preparation is a lifesaver. Shadowstep provides a very useful tool to add to your belt while also making grinding multiple mobs just a little bit more bearable. You can also use a Dagger to unload an amped up Ambush upon your opponents, and then swap to another weapon for Hemorrhage action. Shadow Dance provides a unique CD at level 60, but it's unfortunately not that useful in a PvE environment where you are able to get into Stealth easily.
If you're wielding slow weapons, you'll typically open with a Garrote or Cheap Shot (both of which have greatly reduced energy costs) and start using your Hemorrhage ability. This will make all of your future attacks hit harder, in addition to buffing any teammates. You can quickly use a finisher and regain the energy cost via Relentless Strikes, and then start using Hemorrhage again. You'll be pushing more buttons per minute if you choose to level as Subtlety.
If you prefer daggers, a case could be made for a powerful Ambush/Backstab based build. Opening with a quick Ambush to deal massive damage while slowing your opponent and then using Hemorrhage -> Gouge -> Backstab can yield you some pretty amazing critical strikes. It will require 3 points into Assassination's Puncturing Wounds, as this significantly increases Backstab's viability (from non-viable to "kinda"). Subtlety does great damage when coming out of stealth, so try to stack as many attacks into that 6 second window of Master of Subtlety as possible.
With several available playstyles to experiment with, levels 51-70 are truly the golden years for rogues. We're only really missing our true AoE with Fan of Knives and our pseudo-Misdirection via Tricks of the Trade, but you won't typically be using those while soloing anyway. Try out each of the talent trees and figure out which one suits you the best. If you're not enjoying one, simply swap to another. Weapons are typically easy to find via quests or the auction house, and so there's no reason not to experiment with a spec that tickles your fancy. Leveling is one of the only times where you can truly spread your wings, as by the time you reach 80, everything seems to take a seat unless it maximizes your DPS.