While we're on the subject of magic items, it would make sense to discuss artifacts a little bit. I've always just known about the category of artifacts from fantasy literature, games like D&D, and Magic the Gathering. Of course there are artifacts, my young mind said! The word itself came to mean a powerful magical device left behind by a transcendent ancestor-culture. It never occurred to me that the word had come from a different context. Yes, I knew that any relic could be an artifact, but Artifacts themselves, the fantasy ones, had a special semantic content. The word was (and still is, though I now recognize how strange that may be) charged with power. IT had meaning. Artifacts are things like the Rings of Power or the Tain (Myth) or the Tomb of the Forvalaka (Black Company) or even the Silmarils. They are dangerous, they are ancient, and they are beyond modern ken.
Of course, that's how AD&D described them as well. The Book of Artifacts, while filled with artifacts that I personally would never use, has at its heart the conception of these ancient devices. The guidelines for making artifacts are helpful (though the extra powers can sometimes seem a little silly—why should any artifact be able to produce magic missiles three times a day?) and have provided help in my own designs countless times. Not that I have used that many artifacts in play; sometimes I treat powerful magical items the way AD&D treats artifacts. Not that they're necessarily dangerous, but they are rare and strange and beautiful, relics of some process that the players don't have immediate access to, especially in the levels below 10.
There are certain things that make the notion of the artifact intensely appealing. The first is undoubtedly its prominence in the literary sources, though they take different tacks. Legend and Tolkien tend to coincide, with items of great power not often coming from vanished races or civilizations in these sources, but rather from potent alien smiths and wonderworkers that no mundane man could hope to emulate: Fëanor, dwarven craftsmen in the Norse myths, etc. It seems almost less important that they be actually ancient than simply inaccessible and unreplicable. YOU can't make an artifact, nor can anyone you know. They come from outside the normal sphere of experience.
Of course, there's always a danger of making artifacts simply another class, a highest capping class, of magical item. That danger exists with all magic in roleplaying games. As you codify and describe something that is meant to be ineffable and mystical, it invariably loses these qualities and becomes much more mundane. Artifacts must be handled carefully, sparingly, and with a great effort at making them more mysterious than other types of magical items. Remember, of course, that playing in the RAW even your every day magical swords become objects of mystery as you don't really know exactly what they do (since Identify is a singularly unhelpful spell when it comes to, say, determining exact powers of items). This helps to keep the cloak of magical wonderment in place. Still, artifacts require extra attention in this area.
Full and detailed histories, vague outlines of powers, magic that operates independently of players, and keeping tight wraps on exact mechanical effects can all add to this sense of wonder. The most powerful tool at your fingertips, however, is also the easiest: don't let artifacts become commonplace. I've used a handful of artifact level magics in my time as a DM, so when they show up players tend to go berserk. HOLY SHIT, they exclaim! THIS THING IS A GODSDAMN ARTIFACT! And that's exactly the response you want when someone finds an Earthtiller Staff.