What, exactly, is it that makes an item impossible to use when you do not meet the requirements for it? Does the item cause insufferable pain to the wrong user group? Does it bounce against some unseen barrier when you try to put it on? Are you able to put it on, but simply not to make use of the enchantments? Or, something entirely different?

It could be anything. Usually, the item's description will tell you.

Class specific (Wizard only for a staff) might be simply because others can't figure out how to use it, or don't have the power to.

Race specific (Giant only Axe) might be because it's too big to lift for others.

Alignment specific might be because the item is so evil/good that it pains someone of the opposite alignment to wear it, or rejects someone of opposite alignment.

I'm sure there are other reasons. There is no one single reason why you shouldn't use any items you can't.

"Common sense applies", thus. :) I was mostly curious about the more abstract ones like good-evil, Ranger-only move-silently boots (compared to other, free-for-all move-silently boots), Halfling only short-swords, and so on.

Stuff like Baby-Skin boots, which is presumably wrought from the flesh of sacrificed infants, would likely carry an "Evil Only" descriptor.

This is because -only evil people- would wear baby-skin boots. If you suddenly decide that you -are- the type of person who wears baby-skin boots, please contact a DM and we can probably shift your alignment accordingly.

-Cross

Things like ranger only items are maybe because only a ranger would feel comfortable in enchanted crocodile shoes due to their being in the rough more than the other classes, and thus be less likely to blister on their feet.