Kill Order is Weakest to Strongest
This tends to be unintuitive to most players who are new to MMOs. You get into combat and there's this big, bad dude with a million health wailing away on your tank and you think, "That's clearly the biggest threat, let's kill him first," right?
As DPS, the big bad Champion Boss is your LAST target for two reasons:
1. Killing him will take forever
2. While you're beating on him, all the little guys are violating your healer
The way aggro works in TOR is that, if nobody has hit a mob yet, it's going to beeline for the healer as soon as the first heal goes out. While it's the tank's job to try to round up as many as possible, there are a lot of big spread-out pulls and adds joining mid-fight that make this impossible. It's the job of the DPS to kill those little bastards, or at least draw their attention off the squishy healer and bring them over to where the tank can pick them up.
The reason the weakest enemies are killed first is that it reduces the overall damage incoming on the group as quickly as possible. If you have a pull with one Elite with 50k health and nine normals with 5k, you can reduce the total damage done to the tank by more than half in just a few seconds by blasting those normals into jelly straight away. Focusing on the 50k elite and ignoring the normals means the tank is taking five to ten times the amount of damage they could be if the DPS was killing the right things.
Kill order is Weaks > Normals > Strongs > Elites > Champions/Boss
Who to Guard
Tanks, this one's for you. There are a lot of misconceptions about how Guard works and who should be Guarded so I'll sum it up:
In PvE, Guard reduces the Guarded target's incoming damage by 5% and reduces their threat by 25%.
In PvP, 50% of the Guarded target's incoming damage is redirected to the tank. ONLY in PvP. This does not happen in flashpoints or ops.
The way healing threat works is different than damage threat, where 1 point of damage equals 1 point of threat, plus any bonuses from tank skills/stances. Healing threat is 1 point of threat per 2 points of healing, so it's halved off the bat. Each heal class has a talent in their tree that reduces threat another 15%+. Furthermore, all healing threat is divided amongst all mobs in an active pull. So if a heal is dropped for 5,000 points during a pull of 10 mobs, the healer just gained less than 200 points of threat on each mob. One basic attack from any tank or dps class is more than enough to keep aggro off the healer at that point.
Lots of tanks in PUGs like to Guard healers because they think it'll help the healer stay alive. In reality, because of the healing threat mechanics, as long as a tank or dps has tapped each mob one time, that's enough to keep the healer aggro-free until the weakest mobs start dying (See above). The only time a healer is going to pull aggro is on mobs that nobody has hit yet, in which case their 25% threat reduction is worth zilch, because if the healer is the only one on the mob's aggro table, no amount of % reduction will lower that to zero.
There are exceptions to this, of course, as a healer who is way overgeared for the content can pull aggro at the beginning of a fight if they drop a huge AOE. There are also certain fights where healers generate more threat such as the second half of Terror From Beyond.
But generally speaking, your Guard belongs to the top DPS in the group, because they are the person most likely to pull aggro from you. If two DPS are roughly equal, Guard the melee DPS, since at melee range you only need to beat the tank's threat by 10% to pull. Ranged needs to beat it by 30% to pull.
Pre-healing and When Not to Do It
Since we're on the topic of aggro: HEALERS! If you're not okay with getting a bunch of aggro straight away, quit loading up your tanks with HoTs before a fight!
If you're HoTing the tank, and they leap in, you better be confident that they'll have hit every mob in the pull in the first three seconds, or the first tick of your heal is going to bring every sonofabitch in the zone coming after you.
Generally speaking, on single pulls like bosses, or small pulls where the tanks/dps can hit everything really quick, go ahead and HoT away. Bubbles are OK! They don't count as healing unless you apply them mid-fight. Trauma Probes are basically aggro magnets and will get your face stomped if there's a bunch of ranged mobs.
Fight smarter, not harder!