This mission rewarded me with 48.600XP through refer-a-friend + 20% Heirloom. At lvl 54.

Awesomeness itself.

Wowhead reports me as having finished all the prerequisites, but not this quest, and it is not available to me. Anyone else having this problem?

This does count for Loremaster of Eastern Kingdoms.

Interestingly, the quest rewards show this as adding 500 reputation to the Horde, but it is an Alliance quest.

Any known side-effects?

You do NOT get 'Mission Accomplished' after you have killed the cauldron lord at Gahrrons Withering and return to Chillwind camp. First you talk to alchemist Arbington(standing next to Commander Ashton Valorfist) to get the materials to make a counter agent for each of the cauldrons.You must abtain a 'vitreous focuser', and one 'arcane quickener' per cauldron( but dont buy 4 quickeners like I did to do all the cauldrons because they all disapeared on the first one and they only last 30 min).I had to go back to get another quickener after every cauldron
The mobs you need to kill are at each cauldron,skeletal undead at Felstone,ghouls and zombies at Dalsons Tears ect.It breaks down like this:

1 vitreous focuser(to get juice out of baddies)

Felstone -1 arcane quickener
-6 osseous agitators(from skeletal undead)
-4 runecloth

Dalsons T. -1 arcane quickener
-5 somatic intensifiers(ghouls or zombies)
-4 runecloth

Whithering H -1arcane quickener
-5 somatic intensifiers(ghouls or zombies)
-4 runecloth

Gahrrons W -1 arcane quickener
-4 ectoplasmic resonators(wraith or ghostly
undead)
-4 runecloth

After you have your mats just clik on the cauldron and load the counter agent to compleat the quest. When you have finished all 4, return to Chillwind camp and talk to Commander Ashton Valorfist. He will give you 'Mission Accomplished'