What's the most important aspect of long-term-quality software?

Just doodling...What's the most important aspect of long-term-quality software? I'll define long-term quality software as some piece of software with a lifespan of many years, over which that software can be extended and changed to suit new needs without compromising quality.

Some potential answers, with my best spur-of-the-moment arguments:

• Strong typing - not just variable types, but any sort of type, like a database table. If something changes in the contract (field name changes, or a 1-1 relationship becomes 1-many), then I should be able to make the change and within minutes, know each of the places that are impacted in the code. The justified fear of making changes to a system is driven by the unknown impacts. If I know all of the impacts, then I am in a very strong place.
  
• DRY (Don't Repeat Yourself) - If logic is represented in multiple places, then someone will only change it in one, which will automatically create some inconsistency. If you are lucky, then the inconsistency will be noticed quickly. If you are not, then you will only find out later when the damage is done.
  
• YAGNI (You Ain't Gonna Need It) - Software is complex. At some point, the complexity becomes too much for us to fit in our minds at one time. The longer we can defer that point in time, the more maintainable (and learnable) the software will be. There are two distinct types of complexity though - inherent complexity (because the problem is complex) and artificial (unnecessary) complexity. By introducing functionality before we know for sure that we need it, we are creating artificial complexity. Thus, we will reach the point of too much complexity before we should have.
  
• Minimized Coupling - The complexity of software is directly related to how big it is. When we couple things together, we are making something more monolithic, and thus harder to understand. We also cross a line that is difficult to un-cross. (One coupling-point is just the first of many). Minimized coupling is an antidote to complexity.
  

I think minimized coupling is perhaps the most important, because it has such a direct impact on complexity. I looove DRY though - it is addictive once you try it in earnest. Strong-typing is of limited use without DRY. YAGNI is good advice, although some take it too far.

Are there other candidates?