Rogues are very unique class when it comes to leveling that only ramps up as levels come. With focus on weapons, Heirloom items are very helpful. Without any built in sustain, your focus is on stealth, damage and cc, to survive and kill enemies. Out of all rogue specs, Combat is best because of its reliance on weapons, big cooldowns and access to occasional cleave to take on few enemies and even some elite mobs.
The Combat specialization is what we recommend for leveling in Wrath of the Lich King. Out of all 3 Rogue specializations, Combat is very well-balanced, having strong offensive abilities such as Blade Flurry to take down multiple enemies at once, along with Adrenaline Rush and the new talent Killing Spree. Used alongside other abilities such as Riposte to disarm your enemies, this makes Combat a fantastic leveling specialization.
It is recommended you use a very slow main-hand weapon for large hits with attacks such as Riposte and Sinister Strike and a fast off-hand for Instant Poison procs. Having a fast off-hand weapon is also required once Combat Potency has been talented as it has a chance to restore energy on successful off-hand attacks. Hack and Slash is recommended which gives benefits when only using Swords or Axes, so keep this in mind while leveling and upgrading your weapons.
Once you have talented into Savage Combat be sure to use Wound Poison or Deadly Poison in your off-hand to apply the corresponding poison debuff to the monster in order to gain the benefits of Savage Combat. A Poison must apply a debuff to the enemy in order for Savage Combat to work. Continue using Instant Poison in your main-hand.
Sinister strike > Slice and Dice > Eviscerate or Kidney Shot.
Make sure you’re always maintaining Slice and Dice as a large majority of your damage will come from your auto-attacks and Poisons. When battling elites you can use Rupture instead of Eviscerate. Rupture is generally more damage than Eviscerate assuming it lasts its full duration.
When engaging multiple enemies at the same time you will include Blade Flurry.
It’s recommended to use Blade Flurry when taking on between 2 and 4 enemies at once. Remember to also use the Blade Flurry cooldown with other abilities such as Adrenaline Rush, Killing Spree and also Evasion for faster damage output as well as reducing the damage you take.
Hit Rating > Expertise > Agility > Strength and Attack Power > Haste Rating > Stamina / Critical Strike Rating > Armor Penetration
Note that some stats aren’t obtainable until higher levels, but the above are the general stats to prioritize while leveling from 1 to 80