After quests have been completed, or during the course of completing them, your Player Characters might pick up new armor or weapons that you have put placeholders in for. What is the best way to manage how much damage these weapons inflict, or how much extra protection the armor gives the player?
In terms of enhancement bonus, players should start finding +1 gear almost immediately, +2 gear around level 5, +3 gear around level 10, +4 gear around level 15, +5 gear around level 20, and +6 gear around level 25. Note that any armor that has a high enough enhancement bonus to be masterwork should always be the best kind of masterwork allowed for its enhancement bonus. Masterwork armor is a math fix for the way player defenses tend to lag compared to monster attacks as party level increases; it is not something special that players should have to work to get.
In terms of special abilities, weapons that give a conditional bonus to damage (against enemies of a specific type, against enemies larger than you, on your attacks with a specific keyword, etc) usually give an item bonus equal to their enhancement bonus; weapons with an unconditional damage bonus are very powerful.
Armor and neck items that directly give extra protection are unusual and rather powerful; most give a daily item power, a skill bonus equal to the item's enhancement bonus, or both. If they do give a protective boost, the most common is a +2 bonus to 1 or 2 defenses when a common condition is met (you shifted this turn, you're a druid in beast form, etc) or against certain kinds of attacks (close attacks, ranged attacks from 5+ squares away, etc).
Remember: damage bonuses should go up as the party gains levels, defense & attack bonuses should not (except for enhancement bonuses).
A good item gives one, maybe two of: a very small always-on bonus, a moderate conditional bonus to damage/attack/defense, a moderate always-on boost to initiative or 2 skills, a daily power equivalent to an encounter utility power or capable of making an encounter power closer to a daily power in strength, or an encounter power equivalent to an at-will utility power.
If you have DDI, an easy way to estimate how good items should be is to look through the compendium and see what magical enchantments already exist for that item at that level.