If we are looking for a central concept or theme to this impressive work, I think it has to be an examination of the role of its central character, Maggie Tulliver, and the way that she is always shown to be in opposition to the wider society of which she is a part. The society in which she lives is constantly shown to be conservative, restrictive and narrow-minded, and Maggie's stubborness and the force of her character means that she is always at odds with it. Note how even as a child this theme is indicated through the way in which Maggie definitely does not conform to the image of a traditional girl. She is unkempt, messy and far too intelligent, so much so that her mother despairs of her and her father doubts that she will ever marry because of her intelligence. She is deemed to be "unnatural" by her parents and it is wondered how she will ever find a place for her in society.
As she grows up she has definite ideas about her own wants and desires and achieving freedom. This stands in stark contrast to her brother, Tom, who represents the opposite, as he seeks to live his life in accordance with the expectations of his family and society. The conflict that emerges between them leads Maggie into a crushing internal conflict as she has to face the conflicting urges of either yielding to societal forces and doing what others think she should do with her life, leading a profoundly frustrated life as a result, or to ignore her family and society and do what she pleases, becoming isolated and shunned. This internal conflict becomes so intense that the only possible ending is her own death, which incidentally allows her to be reunited with her much-loved brother.