What do most healers do? They heal. Someone takes damage and they heal it. This is not just boring, it's also cruel, putting the tank through endless torment while never allowing them any reprieve. Those healers which mercifully allow them the sweet embrace of death are insulted, called noobs.
What does disc do? Well it does still heal, but it does it in a better way. If you think that disc shields, you wrong. There's no such thing as shielding. There's a reason it's thought of as proactive healing: that's what it is.
Here's how it works: A disc priest casts power word shield, let's say it absorbs 3k damage (it's a lowbie). The tank then gets hit. Damage is absorbed, right? WRONG! Instead the priest goes back in time and uses future knowledge to perfectly time a flash heal. The result is that the tank appears to take no damage because the heal is perfectly timed. Think of it as giving a blood transfusion the instant someone gets shot.
TL;DR: Disc priests are cool because they can travel though time.
9 hours ago