what I can get knowing the key id ?

Suppose a script knows the key id of another object (different from the one containing the script).
What property of the other object (pos, rot, name, etc.) the script can fetch directly? (I mean without having a script in the other object communicating to the first script)
What change if the other object has a different owner?

Not a whole lot, actually. You can target it for things like llPushObject, but only if it is physical, and you're not over no-push land.

You can get the object's owner's key, and you can get some info on the person from that.

You can send it emails, but that won't do anything unless it checks for them.

You can llGiveInventory other things to it, if you own it, and have mod rights to it. You can also llRemoteLoadScriptPin running scripts into it, if the PIN is set and you have mod rights to it.

Outside of those things, there's not much more information you can get about the object itself, nor are there any properties you can set for it.

Thanks a lot Talarus.