40 spell damage to weapon will not add 40 damage to your spells. Spells follow set coeffecients for spell damage. I HS has a 5% coeffecient. This means that +40 damage will give your holy shield 2 more damage(per block). Generally spells follow set coeffecients based on their casting time, but a lot of them follow their own mehcanics. For the basic/generic coeffeceients go here:
http://www.wowwiki.com/Spell_damage_coefficient
Strength gives 2 attack power(AP) per point of str. AP adds a modifier to your white damage done based on attack speed. AP increases damage per sec done which increases your white damage by a set amount based on your weapon speed. Weapon damage is the raw white damage you do with autoattack and some special abilities (examples being tihngs like MS or sinister strike that do damage based on weap dmg + additional damage).
As far as threat goes, spell damage is far superior to str/ap/weap damage primarily because of Righteous Fury. Your biggest source of spell damage is your weapon, likewise your biggest source of melee stats would be your weapon. It would take a hell of a lot more itemization points to achieve equal threat generated via melee damage to threat generated by spells.
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Tankadin of <I Heart Bunnies> Tichondrius
http://pwnmyheart.com/
http://www.wowarmory.com/character-sheet.xml?r=Tichondrius&n=Echa