Using Starfall effectively in PvE
Step 1 Is Starfall off cooldown?
Step 2 Press Starfall
Step 3 ???
Step 4 Profit!
All right, so maybe it is a little more complicated than that, but for the majority of the time, this is how you use Starfall. I am being totally serious about that. Due to using the Glyph of Starfall over Glyph of Insect Swarm, it is highly imperative that you make the most possible use of that glyph every opportunity that you get. Every second of delay in using Starfall is a second that the DPS boost of the Glyph deteriorates. Deteriorating is apparently a bad thing, too, so we don't want that to happen. There are a few exceptions to this.
If you happen to be running with specific trinkets such as Dislodged Foreign Object, then is it worth waiting a few additional seconds in order for the stacks to build up. Specifically for the DFO, you'll want to use the ability around the 12- or 11-second mark, not the 10-second mark. Trying to hit the 10-second mark exactly can be slightly cumbersome depending on where you happen to be within your rotation, adding in the risk that you could potentially miss the mark and having your last set of stars fall without gaining the buff. Do not try and wait for your trinkets to proc or fish for/force Omen of Clarity procs. Starfall should fall within your rotation naturally and is actually very good about proccing such things all on its own. Only delay using Starfall if you already have these procs up.
A side little tidbit: When starting an encounter, you can use a Potion of Wild Magic just before the pull starts and then toss out a Gift of the Wild while getting into position. Following this up with a Starfall will produce a disturbing amount of burst DPS as you get into the flow of your rotation. Be extremely careful about doing this. The combination of effects is going to put out ridiculous levels of threat right out of the gates and you need to be sure that your tanks are ready to handle it.
That aside, as I mentioned, there are a few exceptions to where you may or may not want to use Starfall right as it comes off cooldown. These are as follows:
The Lich King
The Lich King is an amazingly fun encounter but also a very complex one that has a lot going on within it. Starting out in phase 1, there are going to be a lot of mostly insignificant adds that spawn called Drudge Ghouls. The ghouls will usually be picked up by your main tank and are fairly ignorable. The amount of damage that they put out is for the most part inconsequential (they hit me in Moonkin Form for around 3,000 damage). Even though their damage is ignorable and a balance druid is certainly capable of tanking a few of them at a time, you do not want to pull aggro on them. Pulling aggro on one or more of the ghouls means that they are not going to be next to the tank. This is bad news bears. In pulling them away to yourself, you are preventing them from actually being killed by "accidental" AoE damage such as from Wandering Plague, Chain Lightning or Heart Strike.
Depending on your raid composition, it is also possible for Starfall to gain aggro on the Shambling Horrors that spawn in this phase. If your tank doesn't have Misdirect or isn't a death knight able to use Death Grip, then it's possible for you to gain aggro as the mob moves towards the tank, depending where it spawns. There are no words to describe how bad this is. Pulling aggro on the shamblings not only puts yourself in danger (they will one-shot you) but everyone around you, because they do a frontal cone Shockwave attack. Watch the spawn timers for both of these mobs and be very careful about using Starfall right when they come out. Waiting a few seconds for the tanks to get solid aggro on all of the mobs is more than worth it.
The other tricky part of this encounter is during the two transition phases. During this time, the raid will be on the outer circle of the platform and have to contend with Raging Spirits that spawn. These will appear a few steps in front of a random raid members (possibly even yourself), and they hit very hard. While they may not one-shot you, they will certainly kill you if you hold aggro on them for any length of time. Using Starfall during this phase is very hit or miss. If the spirit spawns in melee or in the group opposite of you, then it can potentially work out all right or even beneficially for you to gain temporary aggro on them, as they won't be in range to attack you and the tanks should have enough time to pick them up. However, if they spawn from or near you, Starfall can easily lead to your death. I've grown to lean on not using Starfall during this phase, but you can certainly risk it if you have enough faith in your tanks. Though I trust all of our tanks without question, I value my own life way too much to put myself in any form of risk.
The last and final portion of the Lich King encounter to note is during phase 3. During this phase, the Lich King will spawn a large number of Vile Spirits in the air around him that range have to burst down as quickly as possible. Starfall is exceptionally useful for this. Although the spell will not be up for every spawn of the spirits, it is very important not to waste Starfall during a timeframe in which no or very few spirits are up. If all of the Spirits are dead or your group is mopping up the last remaining few, then hang on to Starfall until the next wave is released.
Sindragosa is a decently complex encounter that is all about execution. For the most part, you will want to use Starfall on every cooldown for this encounter; however, there isn't much of a need to waste it during the air phases. Periodically throughout the encounter, Sindragosa will fly into the air and mark two to five players (depending on if it is a 10- or 25-man raid) who will be turned into ice blocks. These ice blocks will need to be burned down fairly quickly, but using Starfall to assist with it isn't required. Straight DPS should be more than enough to handle getting the blocks down in time, and you are much better off saving Starfall for when you can target the frosty dragon herself.
Last we come to perhaps the "easiest" encounter that has a few Starfall tricks in it. Easy is perhaps a subjective term. If you are attempting to down this encounter on hard mode, then I'm certain there are some people out there that wouldn't call it easy, by any means. There are only two things to watch out for when using Starfall for this encounter. First, you really don't want to toss Starfall up right as the adds are starting to spawn. Gaining snap aggro on the adds is top priority for the tanks, and most guilds probably will not have much trouble doing so at this point; however, it's polite to not make things any harder on your tanks than they have to be. Waiting a few seconds for the tanks to grab aggro on their respective adds is not going to kill your DPS, so do be kind.
The major mechanic here than you absolutely must watch out for, though, is mind control. You do not, for any reason, want to use Starfall while there is a mind control up. This is especially true in the heroic version, when you have three people controlled at a time instead of one. These people are going to have to be crowd-controlled, and that most likely means that they are going to be Polymorphed. Breaking sheeps/cats/pigs/turtles/bunnies/whatever is not cool, and your raid is going to say mean things about you for doing so.
On a side note, if you happen to be doing the Deprogramming weekly raid quest, then you might want to forgo using Starfall entirely. It's unlikely that you'll manage to kill Darnavan by using it, but it is possible.