I absolutely abhor the idea of magical items (and, even moreso, mundane items) that can't be used at all until a specific level. Sure, since I interpret experience as confidence, it's possible to rationalize this restriction: you have to be a leader of men, or a leader of an entire barony, to have enough confidence in yourself to even control that item. But it doesn't feel right: unless you "go meta", how would the players know this? How would a sage explain it in the context of the world? Plus, useless items tend to get sold. Doesn't it defeat the purpose of describing a magic item if none of the players ever use it?
However, Brendan brought up the topic of magic items, swords in particular, that have additional powers that reveal themselves at a specific level: a magic sword that adds Fireball ability when used by a 4th level Hero, and Wall of Fire when used by an 8th level Superhero. There should be very few of them, perhaps only a handful; they should almost certainly be artifacts, beyond the ability of any wizard to make.
But instead of linking the additional power to an arbitrary level, perhaps it should be linked to a character gaining levels while using the item. For example:
Black Blade of the Baleful Bubble: When first acquired, this is a +1 magic sword with 1-3 low level powers, plus Protection from Evil.
- Heroic level: When the owner of the sword gains a level, roll a d6: if the result is less than or equal to the character's new level, the sword reveals additional powers: the four standard Wall spells, plus Wall of Thorns, each usable once a day;
- Superheroic level: After revealing it's heroic level powers, when the owner gains a level, roll 2d6: if the result is less than/equal to the character's level, the sword reveals its ability to stop time in a 30-foot radius or banish one enemy to imprisonment deep in the earth (Forlorn Incistment,) each once per day.
- Curse: Once awakened to the superheroic level, make a level check with 3d6 each time the character gains a level without performing an act that fulfills the sword's special purpose, whatever that may be: on a roll less than/equal to the character's level, the sword imprisons its owner in the depths of the earth.
This gives a feeling more like "the artifact is getting to know you" than some kind of arbitrary link to system stats.