Spell hit leads on a one-minute single-boss fight at 12.8 Spell damage for 1 %, then casting speed at 12.4 and spell crit at 8.86. A point of Nature damage is worth 0.64, Fire damage 0.33, Intellect -0.16 and MP5 0.10. The weights use Spell damage as the unit and start from the benchmark set, so read each one as the value of a small upgrade on top of that gear.
Per point
- Spell damage1.00
- Nature damage0.64
- Fire damage0.33
- Mana per 5 s0.10
- Spirit0.10
- Intellect−0.16
Per 1 %
- Spell hit12.8
- Casting speed12.4
- Spell crit8.86
Hit
Against a level-63 boss 17 % of your spells miss when you have no hit, and the last 1 % can't be removed, so the cap is 16 %. 
Tidal Focus gives 5 %, and with it this set's sheet shows 8.0 %, which leaves several percent of your casts missing. Each point you add lands more of everything you cast, so hit outweighs the rest. A Tauren gets 1 % more from Endurance. The 17 % and the 99 % ceiling are Classic's. Forever hasn't published a spell table yet.
Casting speed
Haste fits more casts into a fight. On a short fight that is all damage, but on a long one each extra cast also spends mana you will miss at the end, so casting speed drops from 12.4 on one minute to 8.93 on three. No item in this set has it. Your only sources are a Windshaper's 1 % and a Troll's 
Berserking.
Crit
With 
Elemental Fury a crit deals 200 % damage instead of 150 %, so each point of crit is worth twice what it would be to a caster without it. Lightning Bolt and Chain Lightning get 3 % more from 
Call of Thunder, and everything gets 5 % from 
Thundering Strikes. Crit is rolled after hit, on its own, so no crit cap applies. This set's sheet shows 16.6 %.
Spell, Nature and Fire damage
Spell damage is the reference. Nature damage only reaches Lightning Bolt and Chain Lightning, which deal 46.8 % and 14.3 % of your damage, and Fire damage only reaches Flame Shock, Lava Burst and your totems. Weigh a school item by the matching weight.
Intellect, MP5 and Spirit
These are mana stats. Intellect also gives crit, at 59 points a 1 %, which by the crit weight comes to well under a point of spell damage. Their worth follows how short of mana the fight leaves you: against three bosses for three minutes, Intellect reaches 0.62 and MP5 0.72. On one boss for a minute neither counts for anything. Intellect even reads slightly below zero there, which you can treat as zero.
On every fight
Hit leads on every fight. Casting speed is level with it on one minute and sinks toward crit's weight on the long fights. The mana stats rise only on long fights and against three bosses, where the rotation runs dry.