I have a 70 Resto Druid, and I am currently raiding BT (8/9) and MH (5/5),
and I am using the trash-drop cloak from BT.
I enchanted it with +15 Shadow Resistance, and this sate gave it a score of 0.
You suggested I should "upgrade" to subtlety or +120 armor.
In the following post, I will explain why Shadow Resistance is more viable as a non-aoe healer than any other cloak enchant, including subtlety or 120 armor.
1. +120 Armor will only give about an additional 1% absorbed physical damage.
2. Bleed effects ignore armor. So this will not be affected by an armor enchant (Bloodboils for example)
3. The most physical school damage a healer is likely to take is when taking agro and receiving weapon swings from the mob/boss. And 1% extra is not going to save you when you have under 10k hp and some boss lands a 20k crushing on you. Meaning you might as well go for Subtlety to make sure you never end up in this situation to begin with.
4. Subtlety decreases threat done by 2%, but so does the meta head gem for healers. But seeing as a healer is likely to do some overhealing now and then, and overhealing does not give threat, a large chunk of our spells will generate very little threat. Especially when you count in the fact that 2 effective healing = 1 effective threat. And as for druids in particular having the Subtlety talent that reduces threat generation by 20% - with the addition of the meta gem amounting to 22% threat reduction means that when you spam lifebloom that in the end (at 3 stacks) ticks for about 900-1k you will be doing 390 TPS on the target you heal assuming there is no overhealing whatsoever. Now if you can find a tank at BT/Hyjal/SWP level that generates 390TPS then you should start by rebuking him and not the healers for taking agro off him.
5. So now that we have established that healers (Druids in particular) do not have threat issues nor problems connected to physical damage taken in raids, what else is there?
6. This brings me to Shadow Resistance. A dead healer is a useless healer. But so is a feared, silenced or charmed healer. And guess what? Silences and fears and charms and so on are all in the Shadow damage school. So by adding +15 passive shadow resistance you will increase your chance to resist effects that will keep you from healing. Meaning you will - on the average - be more reliable in situations where the rest of the healers might be silenced or feared, like when fighting Azgalor or Archimonde or any other boss that does shadow-school spells for that matter.
So feel free to up the rating for SR on cloak to a 10 score rather than a 0, and remove the armor enchant while at it.