Item level or Intellect value should not be understated, especially early in an expansion. Very often, if it's a significant item level upgrade (13+ item level) then it will be a DPS upgrade to wear it regardless of the stats. As always, sim yourself with Raidbots.com for an accurate assessment of what gear to put on.
In Patch 10.2 all stats scale well with Demonology’s toolkit, even versatility is back as a relevant stat and should be overlooked. The only change that early sims seem to indicate from 10.1.7 is that haste stacking is not as powerful as it used to be for Demonology. Try to lean into a more balanced stat setup if you have the choice without lowering item level, and use enchants, gems and weapon runes to add to whatever stat you are in need of.
Demonbolt Builds (Sacrificed Souls + Fel Covenant talented):
Intellect >> Crit = Vers > Haste > Mastery
Intellect > Mastery > Crit = Haste = Vers
Intellect is just a flat damage increase to all your abilities.
Ultimately it just comes down to damage per point, but here is an outline of what the stats do and how they interact with Demonology’s toolkit.
Haste: Increases casting/attack speed for both you and your pets. This is especially relevant for imps as they have a set amount of firebolts (6) and will time out faster with more haste. To offset this, you summon imps faster with more haste so it evens itself out in terms of rotational gameplay.
Mastery: Mastery increases all of your pets damage as well as implosion and Bilescourge Bombers (if talented). With the amount of pet damage demonology currently has at its disposal this is a very powerful stat that should be prioritized if possible.
Versatility: Gives damage to all your abilities, and half of that damage bonus as damage reduction. In some situations you can choose to use a higher versatility gear setup to survive certain mechanics, although I doubt with the current state of warlock defensives, that this will be an issue outside of PvP.
Critical Strike: Crit does not interact with Demolology’s toolkit in any meaningful way and is just a chance to deal double damage. It is a more sought after stat for Demonbolt focused talent setups as the spell doesn't benefit from mastery.
The best way to calculate stat priorities for your character is to "sim" your character. Method Raider Jaerv has written a series of Metafy articles to help!How to Sim in World of Warcraft
10.2 Note: The recommendations below are from the previous patch, for 10.2 we are waiting on sims to confirm.
Horde: With the heavy focus on nether portal cooldowns with trinkets, Troll is the current best dps race in 10.1. Orc, with its 1% pet damage and 2 minute on use intellect buff is a close second.
Alliance: Dark Iron Dwarf is the best Alliance race for the same reason as orc for horde. Main stat on use is a strong effect to pair with cooldowns. Gnome comes in second place with a 1% passive haste racial.
Weapon: Sophic Devotion
Chest: Waking Stats
Legs: Frozen Spellthread
Boots: Plainsrunner’s Breeze
Rings: Devotion of Haste
Belt: Shadowed Belt Clasp
Intellect/Haste: Inscribed Illimited Diamond (1) is the safe option, but other gems can be slightly better depending on your current stats.
Haste/Mastery: Radiant Alexstrazite (might change depending on current stats).
Augment Rune: Draconic Augment Rune
Weapon Rune: Howling Rune
Healthpot: Dreamwalker's Healing Potion
Additional Consumables: Pepsi Max for 5% DPS gain!
Phials (Flasks): Iced Phial of Corrupting Rage got buffed with 10.1 and now requires 400% of max hp to be taken in order to trigger the negative effect, greatly increasing its uptime. It is very likely that this flask will end up being the best choice. Phial of Elemental Chaos is a safe choice, but Phial of tepid versatility is not a poor choice and offers a small damage reduction as well.