What exactly does Essentials do different with magic items, in comparison to the original core rules of 4th edition?
There's a few things that Essentials changes if you use the rules from their version of the Adventurer's Vault. Naturally, we'll find out most of them when it comes out:
1) All items are classified as Common, Uncommon, or Rare. This supposedly will also include every existing PHB and AV item, with nearly all of them being classified as Common or Uncommon.
2) A Common item will be found 50% of the time, and is an item that they say 'you apply a stat change to your character sheet, and forget about the item until you replace it'. This includes things like a Belt of Vigor that increases your healing surge, or other items with no daily effects or other triggered things. They sell at 20% base value, and large enough cities will sell them to people.
3) An Uncommon item will be found the other 50% of the time, and is meant for the more complex items that have daily effects, or the like. A Power Jewel, which lets you recharge an Encounter power once per day, would most likely fit into this class. Uncommon items, when sold back, sell at 50% of their value. Uncommon items cannot be purchased without DM permission--and usually purchasing or finding one of your choice is something that creates an adventure, rather than being routine.
4) A Rare item will be found once per tier per character--if lucky. These are a higher class of items, with stronger effects than what current items do--the rarity restriction is meant to overcome the fact that they're noticeably more powerful than another item of their same level, and a character would likely hold onto it for several levels before begrudgingly letting it go. When sold, a Rare item gives back 100% of its value. Rare items cannot, and should not, be bought in stores, and finding one is the culmination of an adventure, or a very lucky happenstance.
5) Daily magic item uses are out the window, as now you have Uncommon items being the items that would use them, and the DM now has control of how little--or how much--he wants to give the players that ability. This does, of course, nerf certain classes (Artificer) if this is used.
We'll probably know more about this in a month or so, but there was a preview article this month about magic items, which was unfortunately available only to DDI subscribers.
To summarize: items are now classified as either common, uncommon, or rare. Common items are useful but limited items which don't have powers associated with them. They sell for 20% of what you'd pay for them, and about half the items you find will be common.
The items we've seen up until now would mostly be classified as uncommon items. Uncommon items make up most of the other half of the items you'd find. The important change here is that you can't just create them if you have the ritual -- they require special components and so on. Sounds like built in hooks for adventure, there.
Finally, rare items are the really awesome items. You'd find one per tier (1-10, 11-20, 21-30). They can't be created or bought normally, and if you did sell one, you'd net 100% of its value. These are intended as character-defining items.
There is a great blog, "square fireballs" that has come up with a better / improved list of magic items, since essentials has a very bad (read short) list of possibilities.