I always thought there should be some kind of physics system added, so you could build some cool mechanical structures, like catapults or draw bridges.
The way I see this working is by adding a few craftable items, I have a small list of what I think would work.
----------
Hinges, use place a hing anywhere and atach another block and that block becomes attached with the ability to swivel up and down (or sideways depending on how you place the hinge)
Long rod, used kind of like the fence you place one block for each segment of the rod you want so you can change size (this has the benefit of being thinner then normal blocks making it possible to create more compact structures)
Rope, can be used to attach objects to other objects you have the ability to hold the end of the rope and pull see the long rod for how to place rope if an object is placed on the end of the rope when the rope moves so does the object)
Pully(sp?), A wheel type object that you can place the rope on the block on the other side of the swivels as the wheel is turned can turn automatically when receiving power from redstone
Basket, holds anything you put in it loosely the object in the basket gets a type of physic system
----------
Or some kind of variation of that(I can see some flaws with my system but I'm certain that if notch wanted to he could easily flush them out), I can see people making amazing things if this kind of system was added, plus it would make the game more interesting for people interested in making machinery(we can already see that these kind of people exist in minecraft, but they use elaborate systems of things like mine carts water and redstone circuits)
With this kind of system it would also be easy to build the shower type thing yahtzee wants, but not simple enough to where you just need to place one block down and a way to power it)
I also see some people interested in elevators, again this system enables that and more, with out making it an automated build process(I think part of the fun of minecraft is figuring out how to do something with the tools you are given).