There's a lot going on in Sula that requires us to read between the lines (or at least to read the lines more carefully). At the level of character, we have to be careful of jumping to conclusions about who the protagonist and antagonist are. We have to be willing to suspend our judgment of characters like Sula, Hannah, and Eva and to try to understand what purpose they serve in the novel. This is difficult when they do things like light their sons on fire and sleep with their friends' husbands.
We also have to be on the lookout for issues of gender and race. Although the characters offer a few direct statements about the roles of men and women, this needs to be teased out through careful reading. It's there if you look for it. The same goes for issues of race. We do see instances of racism and we hear about how some of the characters deal with discrimination, but the issues of race and gender often intersect, and this adds a level of complexity to some already-complex ideas.