[color:"orange"]I don't like "borrowing" ideas[/color]
Final Fantasy VII had 'charms' (materia) before Diablo, and there were more than likely earlier games with the same general concept.
Your idea for charms could be introduced in DD2 without resorting to a rune word analogy. For example, a struggling or lazy apprentice mage could complete creating a charm but not seal it properly. Since there were no apparent issues with a single charm, they took the same shortcut again, but they (or their master) found that the charms interfered with each other when brought in close contact (either the in same piece of equipment, or on the same person). The interference from the charms' magical energy fields could produce either positive or negative effects on creatures in close contact with them (your character).
The charm system would have to be relatively elaborate to begin with to make such a secondary effect worth the effort. There would either need to be several different categories of charms (so various combinations would conflict, be neutral or complimentary) or only special combinations of charms would have an appreciable effect (similar to Diablo's rune words). In the former case, mixing unsealed water and fire charms could decrease the effect of both, while unsealed earth and fire charms might give a strength bonus. If unsealed charms were weaker than their sealed counterparts (which seams reasonable), then multiple unsealed charms of the same type could be more effective than they would be added up individually.
Alternately, perhaps the charm slots could be what normally seals the charm's energy, so there would be no need to have sealed or unsealed charms. The equipment itself might determine if charms can interfere with each other. This would simplify charm availability (though you could also have mages capable of sealing or unsealing a charm for a fee), but restrict the use of the 'rune word' type interference effect.
[color:"orange"]The difference from Blizzard would be that it doesn't effect objects (armor, equipent), but the body/mind.[/color]
Part of what I described above assumes that the charms can also effect each other, which may or may not be desirable or worth the effort to implement and balance. Just having certain special combinations of charms would probably be easier to implement, though might appear to be a blatant copy from Diablo.
