I've seen quite a few forum posts about this sort of thing, and I was really curious about how valuable each one REALLY is when you have a stomping talent like.
A critical spell hit deals 150% of a spell's normal damage. Many specializations have talents to increase how much a Spell Critical hits for, up to double of a spell's normal damage. You can gain Spell Critical Strike with Intellect, or with raw Spell Critical Strike chance provided by certain pieces of gear. 1 spell power = 1.5% crit. 1 spell power increases your base damage from 100 to 101, but with 50% crit, it goes from 150 to 151.5 damage. But 1% crit would increase your damage from 150 to 151. Spell power increases the effectiveness of your crit, and crit increases your effectiveness of spell power. Data taken from TheoryCraft addon for version 1.12.1. DoT longer than 15 sec is calculated as 15sec duration. Channeled or cast spell longer than 3.5sec is calculated as 3.5 cast spell. These coefficients are before talents and gear improvements! Example talent bonus: Shadow Word: Pain - 18 / 3 = 6 ticks.
I played a lot of rogue back in the day, and one of the things i'd read about on Elitist Jerks from time to time is that if you're lacking hit rating, it is in some cases better to take crit pieces over some raw damage, when possible. That being said, I look at mages, whom have no hit gear right now, but, from what I see, a lot of mages are rushing straight flat spell damage gear, over crit.