Generally speaking all our secondary stats are pretty close in value with haste being on top. Because of that you shouldn’t just blindly equip items with haste but aim for a good balance between your stats and high item level. Try not to rely on Stat weights, but instead use Raidbots Top Gear to help with gearing decisions.
Agility > Haste > Crit = Vers > Mastery
In AoE situations or when using Witch Doctor’s Wolf Bones Mastery will become more valuable.
Agility increases your attack power, which is just a raw damage increase.
Haste reduces the cooldowns of your main abilities and increases your attack speed.
Critical Strike increases the chance for your attacks to do double damage.
Versatility increases all damage and healing you do and reduces the damage you take.
Mastery increases the damage of class abilities that deal Fire, Frost or Nature damage and slightly increases the chance for Stormbringer procs. Note that since 8.0 this is now a whitelist and items such as trinkets or Engineering bombs are not affected by this anymore.
The best way to calculate stat priorities for your character is to "sim" your characterFind out how to sim
The differences between races are very minimal, pick whichever one appeals to you. I personally would pick Orc, especially if you like to PvP because of Hardiness. They also have one of the better damage racials in Blood Fury that you can macro to Doom Winds.
Flask: Spectral Flask of Power
Potion: Potion of Spectral Agility
Health Potion: Spiritual Healing Potion
Armor Kit: Heavy Desolate Armor Kit
Augment Rune: Veiled Augment Rune