What constitutes a well-crafted melodic line?

I have begun work on two-part inventions, of course influenced by Bach, and being in the first part of the working process, I am confronted with the problem of crafting strong lines.

Here are some inital thoughts and observations:

Well-crafted melodic lines have some or all of these features.

Interesting rhythmic design.

• A variety of rhythms is one of the keys to writing lines that feel natural.
• I suggest looking for unique and instantly recognizable combinations of rhythms. Let go of obvious ideas as they will be more likely to appear trite and common.
  
• Rhythmic activity can increase when leading to a cadence.
• Rhythmic cells are strong motivic features, even more so than pitches. Remember that when developing your material.
  

Memorable and logical interval contour.

• The DNA of a melody is found in the pitches, and each should be a unique combination.
  
• Each motive or line must be logical. One approach is to find the ideas instinctively and then dissect them to fully understand their interval structure, the inner relationships between the notes, whatever features make the music work well. From there one can adjust pitches and rhythms as needed to achieve a more logical flow between the notes.
  

Limited DNA material
Even within one melodic statement there should be only a limited amount of motivic material. These motives can be transformed in to create endless variety, resulting in "beads" that can be "strung" together to create a cohesive whole. This has always be a basic tenet of composition.

Directive pitches are clear and logical.

• High points in a melody should be properly and logically prepared and achieved.
• Repetition of pitches shoult not be haphazard as they are crucial in achieving a sense of centre in a non-diatonic or tonal chromatic environment.

Inner voice-leading.
This is a manifestation of logic in the structure of a melody.

• High notes and low notes tend to be associated together in our minds. Think Gestalt theory. These are especially important to consider when aiming for a well-structure melodic line.
  
• Of noo less important are the inner notes, which can benefit from thinking in terms of voice-leading.

"But do I need to think of all this, did Bach really think of all this stuff?"
A question all composition students ask at one point or another. The answer is yes to both parts of the question.

"But no one hears these things, why should I bother?"
Of course people hear these things, they just don't have musical erudition to explain or even be aware of it. Consider the following:

People that don't understand visual design in all its forms - print, architecture or whatever - react strongly to what is well structured and poorly to what is not, but they can't discuss contrast, symmetry, repetition or even the rule of thirds.

People react strongly to well-crafted stories, but they won't be able to tell you about a three act structure, a plot-point, metaphore are anything else. And they don't need to, they are users not creators.

The brain is wired to seek structure, patterns, and we find beauty in them. Think of this: we straighten the frames in our house, the magazines or utensils on a table, and we do this instinctively. It is our nature.

So why bother? It is your job as a composer to care how the brain naturally react to things when you compose, and as a result to always infuse your music with a perceivable structure and logic. To do otherwise is either lazines or ignorance or both and will always create inferior music.