Losing dos while moving is one of the most intruing aspects of playing range dps. Every step you take thats not needed makes it a dps loss. Unfortunaly that is not how it workd in WoD for all classes, i hope it will move more in that direction, it did abit with WoD from mop and cata, i didnt even bother to PvE in those expansion, way to faceroll.
Frost mages are quite mobile (moreso than fire), but no one plays them right now...
I like my shadowpriest with Clarity of Power.
Stand still:
Vampiric Touch
Mind Flay
Run around:
Shadow Word: Pain
Devouring Plague
Shadow Word: Death
Mind Blast
Level 90 talent of choice
While you run, the dots are ticking and you can insta-cast MB and SWD as well as whatever 90 talent you picked. Combine it with Body and Soul talent for speedbuff and you can effectively outrun most bad effects or mobs, stand still long to recast VT and throw in a MF as needed, and then move on again.