Loose coupling + high cohesion = separation of concerns = single point of concept / responsibility / maintenance.

For OO code the SOLID principles are a good place to start.
http://butunclebob.com/ArticleS.UncleBob.PrinciplesOfOod

My personal favourite is a project I have had to maintain where quite a number of pieces of functionality have been clearly copied and pasted between files, with maybe the odd minor tweak.

It astounds me how often these basic principles are forgotten, especially when deadlines are looming on a project. Unfortunately this generally leads to higher maintenance costs.

Is it possible that mentors are not teaching these principles any more? Has developing now become more about being able to use certain tools?

When a developer encounters someone elses “spaghetti code”, their initial reaction is generally one of disbelief of the mangled mess in front of them. This however usually transcends into a source of amusement. I once enountered a naming convention of variables that began with I kid you not “aa”, “ab”…. which was even transposed into non-related database field names. Needless to say, the time trade off of debugging over a rewrite would have been excessive.

The SoC methodology, or MVC (Model View Controller) paradigm is by far the best pattern of which most of the popular development library frameworks implement. Initially it can seem daunting & complex to follow, yet once the basics are grasped the development process can be vastly improved.

A simple translation of MVC is;

Model–>Structure–>(X)HTML–>Business Logic
View–>Style–>CSS–>User Interface/Presentation
Controller–>Events–>JavaScript–>Data Flow

For more info http://en.wikipedia.org/wiki/Model-view-controller