Some items can't be equipped together, like these:

Mantle of the Coming Storm +1 armor +1 saves
http://www.gamebanshee.com/showshot.php?/icewinddale/equipment/images/mantleofthecomingstorm.jpg

Ring of Protection 2 +2 armor +2 saves
http://www.gamebanshee.com/showshot.php?/icewinddale/equipment/images/ringofprotectionplus2.jpg

So far I've just grown accostumed to mix-match equipment to see if it works. But does anyone know the criteria for the incompatibility? Bonuses too similar...? Thanks.

You are only allowed to equip one +X armor, etc.. item at a time on a character, so you can not have two Rings of Protection on and so forth.
Icewind Dale Tweak Pack gives you the option of removing this restriction so you can equip as many items as you want on each character.

Oh, that explains it... Thanks.