What are the three most important aspects of Sula?
This question is largely subjective as the "important aspects" of the novel may be defined in different ways. I will approach this question by exploring three important thematic aspects of the novel.
The first is the nature of the mother-child relationship in the novel. The mothers have a sense of responsibility to their children; however, they are not affectionate towards them. In the chapter titled "1923," Hannah asks her mother Eva if she ever loved her children. Eva tells her that she did not, not the way Hannah thinks. Hannah, in turn, tells Sula that she loves her but does not like her.
Next, the mother-child relationship filters into the sense of personal responsibility that is explored in the novel. Eva burns Plum because she feels responsible for his dying like a man; she does not think that it is wrong to murder her child. Later in the novel, Sula sleeps with Nel's husband Jude, but she cannot understand why her actions are considered so wrong by Nel. So the characters' actions challenge standard perceptions of responsibility.
Finally, the characters' sense of responsibility calls into question the nature of love and betrayal in the novel. Nel is Sula's best friend, and she certainly loves Nel; however, Nel feels betrayed by Sula because she slept with Jude without even feeling any love for him.