Because digging is controlled by mouse only now, one of the movement buttons? Down feels the most natural to me.

What happened to the idea of using the "other" mouse button? (left button if shovel is on the right slot; right button otherwise)

Yeah, I guess no one really liked my idea at this point ;) There was the idea to reduce hand slots to one item so you have left and right click for two actions. The downside is that you probably lose other things like sword & shield combo (or you have to come up with something else).
If it's just an alternative function, well you have to think about which key to use ;)

While I see the motivation for some control for that, "double pressing" still sounds like a horribly bad idea. What you want is basically a "second mode" for items, another trigger - That is all okay, people could want this soon anyway - but using the button for the other most simple action you can perform with your mouse and double-assign it in lack of other mouse buttons is a bad excuse for intuitive control.
It's dependent on the order, and thus also dependent on your arrangement of items. "Double pressing" by pressing "L->R" is not the same as double pressing with "R->L" and is also not the same as "R+L", but I have to distinguish at least between the first two variants depending on in what slot I carry my item, and be careful that I don't end up using a wrong one in the heat of the battle - depending on the item, it might have some grave consequences. I'd just hate to blow up my UltraJumpFlint in my hands because I was just a bit too sloppy while changing my mouse press from L to R.

Also, even with a hypothetical, well-desinged control trigger like that, I still can't say I like the idea of having a second shovel-mode like that. I like your example of the teleglove, though - there, such an extra modus makes sense. I press [some other trigger], other stuff happens, it's some funky machine, after all! But what could possibly explain why the shovel needs that second mode - other than "Well - we couldn't think of another way to map it to our controls."?

I'd rather try to spend some more time figuring out the shovel problem then introducing a new (awkward) control for this.
You mentioned the simplicity of landscape remodelling - and yes, you are right. We would not want to obstruct this. But the simpliest method - as it is right now - already leads to the problems mentioned in the initial post. So while were at it, we should take our time to figure out the best way to enable the player to remodel the landscape, instead of inventing some new magical mappings to fit the old method into.
I am, too, a fan of the bucket-method I mentioned earlier. To compensate the need of the extra item, a bucket could fill in much more earth than a single chunk. Also, the bucket could be emptied gradually by holding the mouse button, so you'd get much more fine control about when and where to spill your earth.

>Pressing a mouse button does something different depending on whether you have a hammer or a shovel in your inventory.

Yes, obviously - In any shooter game, I shoot different bullets depending on the weapon I'm holding, that's no surprise. "Activating an item" HAS to be dependent on the item, otherwise, we'd have to map each item on it's own key on the keyboard.
It's more on a meta-layer here, though. We have now the mouse button triggers "L" and "R", each activating the associated item - this much holds true in every game situation. Double-Pressing as proposed, however, would change a buttons functionality: From "Activating Item 2" to "Activating Item 1, but use modus 2 now". That can be a huge difference and may lead to much more frustrating situations than the one debated in the opening post :P

>- The Clonks could automatically drop something when they have the chance to collect something fragile that's about to fall, to make digging out flints save. That something could be the flint itself, actually.

Ugh. Remember that little paperclip from Microsoft Word? It also tries to be helpful by doing stuff without you asking it.
Whenever something control related happens automatically that I don't want I am so much more annoyed than when I have to press a button to get what I actually want. That's the main reason I use Latex over Word f.e. - because No, I dont want you to autocomplete my enumeration!

Spawning earth all the time would make digging a lot slower since you would constantly have to get rid of mud. I am not saying that is a bad thing per se. But do we want that?

> how can we make the earth chunks so good that they can just be always created?

I'm not sure that's possible. What exactly do we have them for anyway? They clearly aren't a solution for earth disappearing - fixing that without making digging a pain is basically impossible. Using them as improvised weapons seems pretty silly as well. This leaves only the terra-forming application - which we might well just implement in another way.

How about, say, having an empty barrel in one hand while digging with the other causes the barrel to get filled with the dirt? That would still allow you to move even arbitrary materials around (possibly even more efficient). It could allow having it as a production base. It would overlap in functionality a lot with the water/lava barrel (that we will have eventually, right?). Someone could even make a nice animation of the Clonk throwing dirt into a barrel on his back or something.