I hate the magic item system in normal D&D. In 3e the items were bland, boring, and the vast majority of them were inferior to a few very specific items that boosted ability scores and shit. Plus there were seventeen slots, plus ioun stones. The end result was that your items were more important than the choices you made when designing your character. In short, terrible.
In 4e, you still use way too many items, and many of them are horribly bland, but at least they add additional abilities instead of just being longsword +1s or whatever. I hate a lot of the new systems, but at least the items are generally more interesting than a hat that gives me +2 wisdom.
I only use a few magic item slots: head, neck, cloak, armor, belt, feet, hands, ring, and weapon. Nine is plenty, though, especially since I prefer items to be a little more useful, though without modifying the basic nature of a character too much.
Nice Items Vs. Game-Changers
A nice item gives you bonuses that are handy, like for example a cloak that gives you 10 fire resistance. A game-changer is an item that changes the way your character operates, like an item that gives you +4 speed. There's a big difference there in how much it affects your character. One helps you out, okay, but another will modify everything you do or change your strategic options in nearly every combat. Thus, game changer. Here are a few ideas for items of each category. I don't bother pricing items in GP value, so you're on your own with that. And suck it up if you don't like "per encounter" abilities.
+10 cold resist. +1 ranged weapon damage.
You ignore damage from the first thirty feet of any fall.
Countering Ward Amulet
When you succeed in a saving throw against a spell, the source of that spell suffers 15 electric damage and this amulet shatters.
Iron-Band Belt: During the first round of combat, you enjoy a +3 bonus to AC.
Life Charm: When you roll a natural 20 on a saving throw, this charm breaks and you heal 20 hp.
Swordstring Charm: You enjoy DR 3/-. When you are hit with a critical hit, this charm breaks and you suffer an additional +20 damage on the critical hit.
Your attacks during the first round of combat enjoy +1 to critical threat range.
Boots of Dancing
Any time you take a five foot step, you may move two spaces. You cannot use this ability if you are wearing heavy armor.
Trident +1. When you score a critical hit with this trident, you may immediately cast a fireball spell as though cast by a 6th-level Wizard, using your Charisma to determine difficulty class.
Staff of the Apprentice
Quarterstaff +1. Your first-level spells enjoy a +3 bonus to their DC. Once per encounter, you may cause a first level spell that you cast to inflict maximum damage. That spell cannot otherwise be modified with metamagic or class abilities.
Your spells enjoy a +1 bonus to their DC, and any spells with short range are increased to medium range.
Belt of Superior Spell Components
You reroll any results of 1 on damage dice, when casting spells.
Half plate +1. Any time you are struck for damage with a melee weapon, the space in which you reside becomes covered in magical caltrops that fall off of your armor. These caltrops do not inconvenience you, but the first creature to enter their space will take 5 damage and cannot move further on that turn, at which point the caltrops in that space crumble to dust. Un-triggered caltrops crumble to dust at the end of the encounter.