I don't know what it is about DK's, i can't put my finger on it, but it leaves me feeling unsatisfied.
It's like alcohol free beer. Looks similar, tastes similar but just doesn't have the same effect
I play the DK and it has enough similarities to the pally to remind me that I'd rather play the pally.
Also as my group are lowbies I couldn't run instances at the right level with the team until 58, if i chose a DK.

Rain of fire is great -but it's a directional aoe and I don't really have a setup that will allow that easily.
When I solo on my lock (71) I use rain of fire if i suspect a rogue is about to try and catch them out.

In group situations I would use seed of corruption. Broke up many a group of alliance on the AV bridge
using that.

Hellfire I never used solo, as it self damages - but with a priest spamming holy nova that seems like it might be great.
I think I've seen holy nova trigger some shielding on those it heals when specced discipline. So i might run discipline
with my priest rather than shadow.