The word 'melancholy' is commonly used as a synonym for sadness, though being Melancholic doesn't involve being sad. This type may be more susceptible to feelings of depression than the others, however, which stems from the fact that they are so seldom pleased with the world around them. They always see something that could be better.
While this dissatisfaction may manifest as complaining or even criticism, it can be used to improve the world, to create significant changes for the better. In this sense, Melancholics ensure that everything in the world around them is of a sastisfactory quality; they make sure that things - and people - work.
Compared to other temperaments...
Melancholic and Phlegmatic are both quiet, retreating, low-energy, generally introverted temperaments. Both are often shy, especially in new or unfamiliar situations.
Melancholic people, however, are inclined to stick somewhat rigidly to their personal values or preferences, and as such can be difficult to please, whereas Phlegmatic people are much more likely to adapt to the expectations of those around them - they are eager to please - so as to avoid conflict or being a bother.
Melancholic and Choleric have a habit of taking things seriously, even being touchy, which can be intimidating to the 'softer' temperaments.
However, while Choleric people are forthright, assertive, even domineering about getting their way, Melancholic people are more likely to simply resist change by others. Cholerics are action-oriented, pushing and raging to get their way, while melancholics are more static, remaining true to their values and sulking when things go against them.
Melancholic and Sanguine are both emotional temperaments, moved deeply by the world around them.
However, while Sanguine people express this emotion openly through clear displays, Melancholics are more introverted in their expression; their moods show through expressive but restrained facial expressions and their choice of words.
This combination of two internally-directed temperaments tends to keep to themselves. They are often conflicted by their natural scrutiny of the world and people around them, and the knowledge that sharing these qualms with others will bother them. As such, their criticisms are generally indirect; written in a private diary, or vented to a trusted ally.
People with this combination of 'hard' temperaments tend to direct their criticisms outwards, and may be seen as argumentative, even acerbic. 'Snark' is very typical of this blend. Not content to keep their discontent private, they are more likely than the other blends to start - and be comfortable with - conflict in order to affect a change; to make the world less imperfect through force.
A combination of the two emotional temperaments results in an almost melodramatic nature. They are more likely than the other blends to project their discontent openly in order to attract attention, and to experiment with more 'wild' or unconventional lifestyles and activities.