Magic power is extremely important to magic users. Not only does it increase the power of the spells, but it also lowers the cost of them.
NOTE: If you plan to make your character a powerful, bushy-browed, spell-casting maniac, you should refrain from wearing heavy armour.
As you can see in the picture, this piece of armour provides bonuses for protection and absorption, but in return it also lowers your mana regeneration rate and your overall magic power. Many of the more advanced pieces of armor have these drawbacks, such as Iron, Bronze, Steel and Aluminium items.