Very first mission. Used the key on the doors in the butler's chamber, and both open a crack not big enough for me to get by. Is this on purpose?

Try to move away a little. :)

just like in real life, the door will not open when your body is in the way.

Well, I move out of the way...they stay ajar and unable to close/open them any further.

EDIT: OK, sorted it, although it's counter-intuitive. I can reclose the door if I use the key on it again, and also the door opens all the way if I use the key then quickly step back to allow it to open. Bloody stupid, but at least I can carry on.

Once the lock is open you won't need the key anymore, put it away and you'll still be able to move the door. Using the key one more time will let you lock the door again, that makes possible some nice trick such as trapping guards in a locked room (provided you find a way to lure them in there).

Yes, what is a bug is that you can "lock" the door with the key whilst it is not shut - which stops it moving on its hinges at all...

After some practice you'll learn where to stand (or crouch) when opening or unlocking doors so that you won't get in the way when the door opens. It's definitely different from more recent games but it will be second nature soon enough.