"The Scarlet Letter" gains its stature as a great work of art because Hawthorne has selected a historical-social issue, and going beyond the historical and social context, has explored and dramatized the issue in terms of its fundamental social, moral, philosophical and psychological meaning and implications - weaving an intricate pattern in which all the themes are inextricably intertwined and proceed from each other. Further, this novel becomes exceptional because it deals with such complex phenomena in a rather short story and a rather simple plot.
To the theme of : "The conflict between the individual and society - in context of the consequences of the transgression of a well-established moral norm of society, by the individual", Hawthorne has integrated the theme of man's struggle to attain equilibrium-a harmony-with his own self, with society and the supreme source of morality (i.e. God - here I'm speaking from Hawthorne's perspective).
Finally, he has further integrated the fundamental (and I think THE most important) theme, which I'd put as: "Man's struggle to come to terms with, and glorify the truth."
Concealing the truth, faking reality- Hawthorne has denounced all these and above all - the man who fakes the truth about his own character.
Note how every event of the story dramatizes all the three themes simultaneously-the integration is impeccable.
Guilt, revenge, love, hatred, rebellion, non-conformity, penitence, the nature of sin, the spiritual and psychological condition of the isolated individual - Hawthorne has delved into the nature of all these aspects of human nature and the human soul, which makes reading this novel a psychological tour de force.
I'd state the plot-theme of this novel as : "The story of four individuals-a priest guilty of adultery-who has concealed his deed from society, thereby earning false esteem and respect of his parishioners; the married woman who partook in his sin and who has been ostracized by the society; the husband of the adulteress, consumed with hate and anger-seeking revenge; and her daughter- a living embodiment of virtue, morality and duty violated - of how their relationships with each other and the society in which they live, help them seek true penitence, glorify the truth-and achieve a proper understanding of their errors, both in terms of their values and their actions."


 

The greatest literary merit of this novel is its plot-structure, which is the most tightly constructed, superbly economized and concentrated plot structure - in fact THE BEST - I've come across - especially in terms of Aristotelian aesthetics, given in "The Poetics". Hawthorne hasn't included a single character, situation, event, scene or description which is not relevant to the meaning and the plot of the story. Not a single word can be overlooked. Every sentence is condensed such that it can be elaborated into pages of explanation.
(I am not including the chapter "The Custom House" which is very boring and can be conveniently ignored and which I myself did not complete - but according to many, it is also related to the meaning of the novel).
Another merit is the terrific symbolism - the deep, dark and wild forest; the beautiful & brilliant rose bush outside the prison door, and a lot more, especially the character of Pearl who symbolizes non-conformity, independence and aggressive rebellion against society .
One flaw of this novel is that it is TOO gloomy - which may repel some readers who'd look for some sparks of humor or wit.
Also, Hawthorne himself says too much, often coming between the reader and the characters of the story.
But I think these flaws are very unimportant in comparison to the merits of the novel.
In conclusion I'd say that "The Scarlet Letter" is one of the greatest novels of the 19th century, in fact, one of the greatest of all times. In a story of less than 200 pages, it will give you moral lessons (which you may or may not accept or approve) and insights into human nature and human life which several books may not successfully deal with. Your library and your literary experience are incomplete if you haven't read this novel.