In addition to be directly clicked by the mouse, these items can be activated by hotkeys or macros. Now with the the changes to the key rings (activated and equipment rings in particular), there are some considerations. The old method of dragging a clickie to a hotkey and clicking it from there, is no longer the best way to use them. The reason is that internally what is being stored is a specific location for the item, and if the item moves from that location (like from the activated to equipment key ring, then the hotkey ceases to function even though the item is still on you). Instead you should make a macro that used the /useitem command syntax with the complete name of the item that you want to click. This will allow you to click the item from any location on your character or key rings, and will continue to work even if you move the item around. (see /useitem)
What follow are the lists of the essential clickies that different classes should consider getting.