Full disclosure! I do not main a Discipline Priest in World of Warcraft. I main what I feel is the stylistic opposite of that, an Unholy Death Knight.
I don't play better than any one else. I play successfully, but that's only because I'm 1/10th of a raid team that is currently 8/12 hard modes in Icecrown Citadel - my efforts are a viable contribution to the whole. I take talent choices that seem "useless" to the outside viewer. Looking and comparing my talent build to the "accepted best" I've made some silly choices too!
However, a lot of guides I see that talk about playing a Death Knight haven't quite been brought up to date for low level Death Knights just yet, so I'm going to give a once over here, talk about some of the basic ideas behind playing an Unholy Death Knight.