Originally Posted by
Tehterokkar
I'm pretty sure it is just straight up +1 damage to spells/heals per 1 point of +damage/healing.
Spells in Pre-Cataclysm WoW were very straightforward. +1 spellpower meant that you did 1 more damage with each spell. DoTs would get a random +1 damage boost on 1 of their ticks. There was no spell power scaling of spells like we have today with spells scaling with "240% of spellpower", it was just flat additive.
There were some random talents scattered across talent trees that gave +% spellpower scaling to certain spells, off the top of my head the first one I can remember is Empowered Corruption(not 100% if it existed in Vanilla) that gave something like 12% of total spell damage as extra spell damage at max rank to Corruption.
Everything above applies to +healing also(as well as +Element spell damage, which Vanilla was full of).