Door locks....is this a feature or a bug?
 

I can lock the passenger side door of my '89 3.2 from the outside with the key but can't unlock it. The key goes in the lock but won't turn far enough to unlock it. It'll lock & unlock just fine when using the knob or the central locking. The driver's door works normally.

Is this some sort of "safety" feature, or is the lock broken?

There is a power locking switch that sits on top of the lock mechanism on the inside of the door handle, maybe it is faulty or the mechanical arm that is on the inside of the door handle could be broken. You should remove the door panel and have a look inside. The door panel is very easy to remove.