Wabi Sabi

There are various translations of this Japanese term having to to with impermanence and imperfection, and that this is the natural state of things.  The use here is about accepting imperfection of both the material itself and the installation of it.  This doesn't imply that one should accept lousy material and shoddy workmanship--in fact it is often a bit more difficult to work with  materials that have "defects--but rather that there are materials that have traditionally been considered to be of lower quality that can make beautiful finished products.  What it also implies is that a product made by a machine looks like it was made by a machine, and one made by hand looks like it is made by hand.  This is not to say that one is necessarily better than another, but rather that one shouldn't reject the made by hand one because it is not "perfect": in fact in the Wabi-Sabi worldview, the hand made one is more prefect.

In construction there are various way to hide imperfections; for example, trim pieces like baseboard and window casings were originally created to cover up uneven joints, not really for aesthetic reasons.  A joint with no trim is much harder to make than a joint with trim, particularly when the two surfaces are different materials.   Even with similar materials, smooth and "perfect" are harder to do than rougher, for example smooth wall drywall takes considerable skill, but you can do textured surfaces much more rapidly.  As with everything else in construction, the devil is in the details as many textured drywall treatments have come to be synonymous with cheap, so care must be taken in which solution you choose.

The natural building takes this idea to the limit and designs many things with curves and irregular surfaces, doing things like using a slab of tree with the bark still on it (often referred to as a "live edge") as shelving and counters.  On the opposite side of the spectrum, the industrial modernist movement also often uses imperfect looking materials, often metal, but uses them in very rigorous way so that the net result is very linear and highly organized, creating a somewhat machine-made type look.

This idea of wabi-sabi can be carried out throughout the house, into the finish trim, doors, windows etc, although clearly it is important that windows and door function smoothly, even if they don't look that way.  This "natural" look gives much more flexibility in using reclaimed materials, since those materials often have marks in them that are difficult to remove.  If one thinks of these marks as "patina", then reclaimed materials are not only easier to use, but you get an added benefit of bringing character into the house that can only otherwise be acquired via many years of use.

As a corollary to this, always build things to last a long time, because they will hopefully have many uses.