The Basics Matter

Gear is important. Using a good ability rotation is important. Knowing the basics of good gameplay is more important than gear and rotation combined.
To put this in perspective: say you have a buddy who is of equal skill on your raid team, you are both DPS specs. If you have equal item level, you will do about the same damage. If your friend has 5 item levels on you, he will do about 5-7% more damage than you. 15 item levels: about 15-20%. This is a rough but pretty close ballpark of the gain you should expect from gear.
If your friend switches to a new and fancy rotation with a ton of complicated tweaks that he found on the internet, he might gain about 5% damage on you.
It is not uncommon in a typical casual guild for the best player to be doing 30, 40, even 50% more damage than the worst player with similar gear levels. Everyone leads you to believe that this is because you chose the wrong talents, gear, or aren't using your spells in the right order. Sure, those things can help... but a huge part of the damage that you are leaving on the table is because of the basics.

Keybindings

Do you turn with the keyboard? Do you click on your spells instead of using hotkeys? These things will significantly hurt your performance. Read our keybinding guide before you do anything else.

Melee: Remember That You Are Melee

Many melee spec players don't realize how much damage they lose by simply being out of range of the boss for a portion of the fight. It sounds like such a simple and obvious thing... but on some fights it can be much harder than you think to stay on the boss.
Before worrying about anything else, practice staying in melee range at all times on each boss fight. There are two things that get most players:
  1. Fire! Learn the boss's patterns so that you can avoid fire and other ground effects while also staying in melee range. Some bosses don't cleave. While it is better to stay behind an enemy, sometimes moving to the front for a few seconds to avoid a ground effect can keep you in melee range.
  2. Learn how your tank likes to do the fight. There's nothing worse than you dodging left and your tank dodging right. Now you have to close all that distance, losing valuable DPS time. Communicating with your tank is essential to high damage. (Fun fact, it takes 1 second to run 7 yards, or about 4 seconds to run from where your ranged players tend to stand to where the boss is. That's a lot of lost DPS time.)

Ranged: Always Be Casting (ABC)

This is the main difference between high and low damage players. There are a few major aspects to this:
  1. Only move when you actually have to move, and only move as far as you need to. Many players move way too often and way too far. Learn the fights you are doing in the game and have a plan for where you will move for each mechanic. After a few attempts, you will start having some idea about where to go and when. Coordinate with your team members so that you aren't forced into a lot of extra movement. Encounters in WoW become much more manageable once you can cut through the initial chaos of a new fight. So many times there is someone off to the side who always seems to be running from something - is that you? As you learn an encounter, focus on one mechanic at a time if you are having trouble.
  2. Practice moving when you can't cast spells. After you use an instant cast spell, you are locked out of using other spells for 1.5 seconds (this is reduced by haste). This is what we refer to as the Global Cooldown (GCD). You should be looking to move while you are on GCD as much as possible. If you need to work your way across a room - do it during your GCDs. Some people call this a stutter step.

The Spell Queue

Make use of the Spell Queue. If you are casting a spell over and over, you can press the button to use that spell a little bit before the current spell is done casting. This queues the spell up to be cast right after the previous spell with no delay. This also works with the GCD. If you press the button to use a spell just before the GCD is finished, the game will cast your spell right as the GCD finishes with no delay. A lot of players don't take advantage of this and end up with little gaps of time between each spell. Those delays can add up to serious losses. Some players spam their spell keys to be able to use them right as they are available. Other players will use a rhythm and press keys during the Spell Queue window to chain spells together. Both techniques work - just find the one right for you. If you have seen someone suggest setting the Spell Queue to something very short or zero, don't listen to them - that will almost always result in a loss of performance.
You can show what your Spell Queue is by typing /dump GetCVar("SpellQueueWindow")
Set your Spell Queue by typing /console SpellQueueWindow 400. Replace the last number with the Spell Queue you desire in milliseconds. We suggest the game default of 400ms for most players.

Everyone: Learn the Timing of Boss Fights

Every spec in WoW has abilities that can only be used a few times per fight. These are your abilities on 2-3 minute cooldowns usually. Using these at the right time on a fight can affect your total damage dramatically - easily 10% or more.