>That is why the mouse buttons are always reserved for using stuff your are interacting with and [space] is for interacting with something.

I guess it's a matter of perspective. If I want to complete the task "bring tree to sawmill", there is no obvious reason for me why i would assume that one part, "chopping the tree" is "using" the object and the "grabbing and moving" is "interacting".

>Obviously you wouldn't want to grab a tree or enter a building while trying to shoot with your bow.

That's why I sugggested the other mouse button. Left-clicking would always shoot my bow, while right clicking would select the object I want to grab/interact with.

>How about a cannon on an airship? [...]

We don't really have many of these situations now, but as I understand it, it'd work like this:
- move lorry to airship
(- press button to release lorry)?
- look for airship button
- press airship button

Using the RMB to interact/grab, it'd go like this:
- move lorry to airship
- Right-click on airship

The real important part is that I don't need to search the HUD for the right button. I can just point and click at what I see. This is important, since the button in the HUD even changes from situation to situation - one time, the airship is on 1, the other time, there is more stuff and it is on 3. Basically, as it is now, players can't really learn how to interact with certain objects, because it could work differently every time.