As you know, your character has a multitude of combat ratings. One set of ratings are specific to spell casting abilities.
These spell ratings are: Spell Hit, Spell Penetration, Spell Crit, and Spell Haste I like to keep things simple, so this will not be a deeply mathematical posting(click the links for the math). However, simply remember the higher the numbers, the better and the higher your character level the less potent the same spell rating will be. For example, 14 Spell Crit. ratiing = +1% Spell crit at level 60 but only about +0.6% at level 70. Different ratings are more important to different classes and in different situations. Hopefully, discussion will follow to illustrate that.
Spell Hit increases a caster's chance to hit with spells. Or rather, the more spell hit gear (and talents) you have the less the chance an enemy will "resist" your spells. A spell that fails to hit is displayed as a resist, although this is different than spell resistance mechanic and leads to some confusion. Your base chance to hit is based on the level difference between you and your target. There is always a 99% Spell hit rating max. Effectively, you want +3% chance to hit for PvP and +16% for PvE/raiding. Some important numbers for a level 70 caster are: 12.6 points of spell hit = 1% chance to hit which means the effective cap 202 spell hit points
Spell Penetration reduces your enemies resistance to your spells. In other words, it overpowers an enemie's spell resistance. Important to remember: 1 pt of Spell Penetration negates 1pt of resistance. Although, Spell Penetration has no effect upon resistance due to level difference. Enemies get a base 5 resistance per level higher than you. It is widely believed that Spell penetration is only marginally useful at this time.
Spell Crit rating is the chance to do extra damage per spell cast. Your base crit chance is a product of your Int stat, increased generally 1% per 80 Int @level 70. Talents, items, additional buffs, spells, totems, you name it can increase this effective number. Base damage increase is 50% but can be increased also with talents. On items, 22 Spell Crit Rating = 1% Crit. Spell Crit is regarded as less "effective" than Spell hit point for point realistically. However, many great and wonderful effects only happen upon a crit. Therefor, the importance varies greatly.
Spell Haste is the rating which determines casting speed. Soon to get a boost in coming patch to be more effective and therefor more desirable. Soon Spell Haste of 15.76 points = 1% increased cast speed. Currently it you need 21 pts to gain 1%. Not alot of significant info is around about this stat. nor is the stat a huge focal point. The reason we likely haven't seen it much yet is because so far, spell haste gear only drops at the highest levels. Currently, most haste comes in the form of temporary buffs that allow you to get a few fast casts off. (Most common to our troll friends.) It is regarded as devastating in PvP, and is where it shines most.
Now, simply speaking, Spell Hit, Crit and Penetration work in symphony when a spell is cast. It is easy to think of spell results in a three step process. (Three rolls for us old Pen and Paper gamers.) First roll, does the spell hit or miss? Second roll, does it crit?(natural 20 woot) Third roll, how much damage penetrates?
Advise is to seek to increase your +damage, +hit, then +crit in order priority (generally speaking).
Please discuss, and/or link to better info.