The wizard is the one of the classes that can benefit from having a full golf bag, but it isn't practical to specialize in everything
Suggestions:
1. Gear to your primary spell power and school
2. Gear to a secondary spell power and school
3. Look to Universal DC, Spell power and Arcane Augmentation for everything else
4. If you are wanting to have a specialty that you use on occasion, such as Transmutation (flesh to stone, curse or knock)
Example for myself:
1. Primary Spell Power: Negative
2. Primary Spell School: Enchantment (I play more as a controller)
3. Secondary Spell Power: Acid (taking advantage of Past Life Spell power)
4. Secondary Spell School: Necromancy (for the stuff that needs to die now)
5. Universal Gear for everything else
6. I have an Thunderholm Orb specifically set for Transmutation Focus - Done for Raids where curse + flesh to stone are the best controller.