Mind spells have to do with priests in general as priests heavily emphasize mental discipline and control (propaganda) this is probably a reflection of the real world where religion/priests are associated with both (in varying aspects, ie for mental discipline think of abstinence or eastern monks and vow of silences, etc).
Devouring Plague is legacy of racials, it was the undead priest racial when all priests had racial abilities. Fear Ward, Desperate Prayer and Hymn of Hope are also left overs from this era, with Fear Ward showing the fearlessness of dwarven priests and Hymn of Hope the inspiration of the Draenei. Removed however were Shadow Guard of the trolls, Starshards of the Nightelves (Tyrande still uses a similar ability, it's very close to Starfall). These abilities were deemed somewhat unbalanceable so they were either removed or given to all priests. (Chastize was also a racial and became Holy Word: Chastize.)
Void abilities do not correlate with Old Gods at all, but rather the absence of light. Think of Muru who was known as a Void Lord before being reborn into a being of light. The Void and Light are two ends of one axis so to speak. Void is emptiness or darkness.
Also, priests abilities use to have a few more "shadow word", for example Silence was called Shadow Word: Silence. And the opposite to these were "power word" such as Power Word: Fortitude and of course we still have Power Word: Shield, Power Word: Barrier and the new Power Word: Solace.
Power Words strengthen and protect, Shadow Words damage and weaken, Holy Words restore.
Lastly, I am not sure where you're getting these "elementals" themes from. There are other elements in WoW (for example there is a Sound Elemental), if you simply mean from resistances, Holy Resistance did exist at one point in time, it was just extremely rare and virtually never itemized. Because the theory goes Holy damage was divine and could not simply be resisted.