One distinctive aspect about this novel is the way that Dickens uses characterisation. One famous distinction that E. M. Forster noticed in the work of Dickens, and which he explored in his work of theory, Aspects of the Novel, is the way that the novels of Dickens literally teem with characters who are, in Forster's words, "flat." He used this distinction to refer to characters who are two dimensional, and have one or two characteristics, and who do not develop or change throughout the novel. These "flat" characters are contrasted by "round" characters who are three dimensional and whom we are given insight into their motivations and actions. These characters change, mature and develop during the course of the novel.
Thus when we explore Oliver Twist, it becomes clear that almost every character is flat. Consider the almost angelic goodness of Oliver and the way he remains uncorrupted by the evil characters around him. Compare him with the devious and devil-like characteristics of Fagin, who remains obsessed with greed and the accumulation of his wealth. Likewise Fagin finds a true companion in Bill Sykes, whose evil nature is predominant, just as Mr. Brownlow continually believes in Oliver. The only character who blurs the boundaries of this good/evil divide is poor Nancy, who nevertheless shows herself to be a predominantly good character but trapped in an evil situation from which she is unable to escape.
Thus when we consider some of the aspects of this novel, one of the features you can comment on is Dickens' use of characterisation to create a morally unambiguous universe, where good characters remain good and bad characters remain evil, with very little blurring of the boundaries between these two states.