The GCD, more commonly known as the global cool down, can be important in figuring out what order to use abilities during DPS. Yes, your Starfall may be off cooldown, but you just procced your Lunar Eclipse. Which is more important in terms of your DPS? This is not an easy question to answer and a lot of different factors can play a role in it.
The first and most important factor is how much DPS will you gain or lose out of doing one action in a GCD versus another. Lets just look at some normal numbers at what occur. These are close to what my numbers and depending on where your gear is, it may or may not be close.
|Ability||Non Critical Strike Hit||Critical Strike Hit||Affected by Eclipse|
|Wrath||8k||15 – 20k||Yes|
|Starfire||8k||22 – 30k||Yes|
|Starfall||2k per Star||2k – 5k per Star||No|
|Treants||800 per hit||1k – 1.5k per hit||No|
Now that we have a base idea of numbers, lets look at different conditions:
The first condition is during no eclipse. This means at that time, the critical strike rating is anywhere from 37 – 42% depending on cool downs, etc. Also, remember, that Starfall is not affected by Eclipses. This would mean the best time to use Starfall would be when no Eclipse is up and there are multiple mobs up. Same thing for treants. They are not affected by Eclipse bonuses. Also, DoTs have the potential to proc an Eclipse, but are not affected by one. In other words, it would appear that the best time to apply DoTs, Starfall, and use your treants are when there is no Eclipse.
The second condition is a Lunar Eclipse. At this point, your critical strike for your Starfire goes up to 100% (assuming you are Lunar Crit Capped). This means that Starfire would give you more DPS even though it is slightly higher than the GCD (My Starfire is currently at a 1.66 second cast time). This means that even if DoTs fall off or Starfall comes off cooldown, it is better to keep casting Starfire because of the damage increase.
The same holds true for the third condition, a Solar Eclipse. Although it is not possible to get 100% critical strikes for Wrath, you can get up to around 60% during an Eclipse. Wrath is also equal to the GCD when you are above 408 haste so in two GCDs, you are likely to get around 35 – 40k worth of damage, which would be better than even a multiple mob Starfall.
It would appear that all your abilities are cut and dry when to use except for the fourth condition, which is moving. If you are moving, you are not able to cast Wrath or Starfire, so you are only left with all of your other abilities. If you have to move during a Lunar Eclipse, you can still use Starfall or treants while moving or refresh your DoTs to get DPS on the boss.
The fifth condition is what I would call situation specific. For example, during phase 3 of the LK fight, you have to DPS down the vile spirits. Even if I don’t have an Eclipse currently and the Starfall just came off cooldown, I will wait until right after the LK calls out the Vile Spirits so that we can DPS the vile spirits down easier by me casting Starfall. Or even if I just procced an Eclipse, the Vile Spirits dying is more important than 100% critical strike on LK. But I will be able to use the Lunar Eclipse on the Vile Spirits as well. And if you want to break my heart, let me proc a Lunar Eclipse and get Unchained Magic at the same time during phase 3 of HM Sindy on 25 man. I have all this power and can’t use it due to the AoE effect of Unchained Magic.
There is also one exception to all of these rules and it is Faerie Fire. Now, in 25 mans, where you will probably have a feral druid and a shadow priest, you may worry about it a little less or not at all, but you need that 3% hit to be hit capped. If nobody else can cast it, you have to. And hitting the boss every time is more important than your 100% critical strike Starfires during a Lunar Eclipse because you will not hit the boss with full effect or may even miss the boss entirely.
So hopefully, this will help you in giving you a guide when to use abilities based on the GCD. There are other factors that play a role, like trinkets, etc. but this should be a general guide for you. And this holds true for all the classes. Everything is based on how much DPS or healing or threat you can do per GCD and how to maximize it.