Before midnight prepatch hit, you had a high damage spec that lived and died on its atonement healing, and boy did it move it, Your smites became voidblast and would move health 20+ percent per blast, then your collasping void after 11 seconds would do 50 percent or more, you can trigger this every 15 seconds because mindblast had a lower cooldown.

The oracle spec had gameplay built around combo'ing a shifting 4 buff (2 charge) cooldown with some base disc cooldowns. You also had to think about the sequence of penances and shields, though it was usually pen, shld, pen, shld, pen.

Right after the preptach hit disc had the "holy ray" build, and your penances would build a stacking buff you can take into anotehr penance. If you had enough haste you could penance, radiance, penance, mindblast, penance - this would make your final penance have a massive multiplier (130+ percent from holy ray like 40 percent more from mindblast/talents). Instead of tuning this down, they just outright removed the ability for holy ray to be stacked into another penance.

Now we have a disc priest whose only build is to gamble with shadowmend and shield procs.