So, despite not really being able to afford it, I picked up the Magic Item Compendium this past Friday from Dragon's Table. (...which apparently has a blog. Who knew?)
I haven't read through the entire thing in detail yet, but what I have read is pretty impressive. Here are some of my initial impressions:
New Item Types
(...which may have been in some supplement that I hadn't seen, but are new to me, at least)
Runestaff - The runestaff is a staff that has no charges. Instead, you spend spell levels to cast the spells in the staff. You can only be attuned to a single runestaff at a time so, while this is a great item for spontaneous spellcasters it isn't utterly gamebreaking and is also very nice for spellcasters who prepare spells, since they can take advantage of their flexibility more.
Augment Crystals - Apparently, this is a Diablo II thing - little crystals that attach to weapons or armor and add magical effects. I have mixed feelings about them. On the one hand, they are great for gameplay: you can have a minor magical weapon with a neat effect at low levels when you'd otherwise be wielding a masterwork or, maybe, +1 weapon. On the other hand, they seem like they'd change some assumptions about magic and the setting. If you're starting a new game, they are probably a good idea. If you are thinking about adding them into a game... it could work, but you might want to be careful (perhaps treat them as a new or lost technology).
Item Sets - The item sets are an awesome idea. They are sets of themed items (usually mostly clothing, but often including a rod or weapon) with cool, related powers. When you have more of them, you gain some additional synergistic powers. My one complaint? They put stupid limitations on the creation of these things. Why? These are the sort of flavorful magical items that I would want my players to think up...
New Charge Mechanics
There are a bunch of items that have a new (to me, at least) charge mechanic. These have a set number of charges (usually 3 or 5) that refresh every day. They can usually be spent individually for minor effects or together for more impressive effects. I should note that I am not a fan of uses per day items unless those items are actually keyed to a daily sort of thing (such as being single-use, but recharged at sunrise or something). I do think that I like these sorts of items, though. They have a flexibility that many magic items lack... and they involve player choice and do more than one thing, both of which are nice.
The Magic Item Compendium realizes that everyone wants certain items - things that provide bonuses to ability scores, armor class, and saving throws. It also, thankfully, realizes that gloves of dexterity and cloaks of resistance are not only bland, but they preclude the use of other items in those body slots (I sort of hate the body slot system as it is now, but that's a topic for another post). As a fix, the book suggests allowing players to add these sorts of bonuses to more interesting magic items, and it provides mechanics for doing so. So, yeah, now your Boots of Striding and Springing can also add a bonus to your dexterity. Rock.
Tables, Tables, Tables...
The end of the book is filled with tables that incorporate magic items from this book and the DM's Guide. Some of the tables are random treasure table type things. Others list items by price. They also have some sort of guide where items are categorized by 'level' - more or less the level at which those items could be expected to come into the hands of the PCs. The last one seems a bit iffy to me - the examples they gave didn't really match up with my preconceptions - but I also didn't look too closely at it.
As I said, I haven't read through the whole thing yet, but if I were running a campaign right now, I think I'd consider this an essential book.