Immune mind affecting
Immune spell school
Immune specific spell
Immune spell by level
Immune Critical Hit
Immune Sneak Attack
Immune Death Magic
Some of the On Hit effects (dispels, vorpal and the slays in particular)
Cast Spell of a spell level higher than 4
Damage Resistance of greater than 15
+5 item or equivalent
..... you may ask a DM to make that item no-drop AND Plot for you. This is completely voluntary. You do not have to participate, and any DM can do this to your items with his or her DM wand. The only stipulations are that the item MUST contain at least one of the properties on the list above, and the item cannot be a weapon. There are absolutely no consequences if you choose not to participate, but there are many advantages to having your item be no drop and plot. For one thing, it comes along with you when you die, and it cannot be stolen or lost by putting it into a bag, because the engine will not let you put no-drop items in bags, AFAIK.
So, if you read this and like the idea, and you have an item that belongs to this category, feel free to stop one of your DM's and ask.