Putting a number to it I would only want magic items at any one time to increase the effectiveness of any character by max 10-20%, enough to notice but not enough to make it essential, or with balancing pentalies. +1 dmg -1 attk is the most basic example I can think of. Effectively Power Attack for free.
Alternatively give certain items feats. e.g. Armor of Bull Rush or a Axe of Great Cleaving.
Alternatively give certain items feats. e.g. Armor of Bull Rush or a Axe of Great Cleaving.