Well, I started a detailed analysis of why 4e's magic items disappoint me, and how it came to be so, but it was really turning into a much longer essay than I was willing to write. Besides, DEFCON 1 hit the most important points. In short, for me 4e really strikes the wrong balance with respect to codifying actions and leaving the door open for creative use of magic items. This spirit of "over specification" is throughout the whole system, but it particularly disappoints me here because the major shortcomings of 3e's magic items were so well recognized.
I also wonder if the novelty and IIRC success of the Magic Item Compendium at the end of 3.5 may have made the 4e system seem like a much better idea than it actually was. (From my perspective, of course. If you love 4e magic items, I have no problem with that.) In part this is because 3e had been starved of almost everything besides the Big 5 for so long (the contrast was quite stark!) that all these new and revised magic items couldn't help but seem awesome. Plus, the reasonable prices created really difficult body slot decisions that were otherwise rare during the entire edition. Finally, the codified nature of many of the 4e-like MIC items was softened by the less-precise nature of 3e, and any existing items with really broad or unusual uses were obviously still available, so issues due to the pervasiveness of that style weren't necessarily obvious.
As others have said, one thing that might improve the mysteriousness of magic items would be to give only partial information about them, instead of a short rest leading to full knowledge. Something like each effect of the item having an appropriate DC to determine its function, and missing by 5 or less at least tells you there is some additional function there. Maybe the first check a character makes on an item takes a short rest, a second requires a couple hours, and after that additional information is beyond reach without some other form of help (a ritual, a piece of lore from a library, explicit experimentation, whatever.) The possibility of powers that were missed, or the certainty that they were missed, could keep players on their toes. And with item rarity added in Essentials, it might be cool sometimes if you don't actually know which items are rare, because some of the powers it holds are so obscure you have difficulty discerning them. Or if every once in a while the wizard rolls a natural 20 on an Arcana check and get the "unknown power" result, that could be a cause for excitement at the table.
And for something practical, here is a riff on the classic immovable rod that I think could be used creatively. I wrote it for a homebrew system, but it converts easily enough, especially since the Immovable Shaft is in the Adventurer's Vault. Maybe someone will find it amusing.
Telekinetic Anchor
"These two interlocking cylinders, one of obsidian and the other of pumice, hold firm or fly."
Level: ~15?
Price: ~25,000 gp?
Wondrous Item
Power (At-will): Minor Action. Separate the two halves of the Telekinetic Anchor. The obsidian half stays in place, exactly as though it were an Immovable Shaft (AV 173). While a creature grasps the pumice half it has fly 3 (hover), otherwise this half simply hovers in place. These effects end prematurely if the obsidian half is moved for any reason, or if the two halves are reconnected (a minor action). Otherwise, these effects last until the end of the encounter or for 5 minutes, whichever comes first.
Special: You can only activate this power if the two halves are connected.
In my original version it lasts 1 minute and is the equivalent of fly 6 (hover), but I'd forgotten how much stuff that lets you fly costs in 4e. Season to taste is what I'm saying.
