The play A Raisin in the Sun has a couple of things that I enjoy every time I read it, so those are the things that I would identify as being done well:
1. Clear but light symbolism: I always enjoy the little potted plant that Mama tries to nurture but that just doesn't get enough light in the cramped apartment. I also always enjoy the hallway bathroom, shared with the other apartment residents. When I teach Hansberry's play, I usually also give students a copy of Gwendolyn Brooks' short poem "Kitchenette Building," which uses memorable details (including a shared bathroom) to depict the less-than-desirable living conditions of some urban black families in the mid-twentieth century.
2. Conflict between generations: I agree with akannan that the play does a good job depicting the conflicts between generations. Mama is an especially important character and representative of the older generation, who insists that life much include a place for God (her conflict with Beneatha really brings out this idea) and that individuals must value a free and moral life over a wealthy one (see her conflict with Walter). I also really like how the play brings in black nationalism and the "Back-to-Africa" sensibilities of the younger generations in the 1960s.
Other people are going to have different answers, of course. Each answer is likely to depend on what each person knows about black history, drama, and their own values.
Hansberry really pulls out the critical issues that are facing families of color in the 1960s and 1970s, and still do in the modern setting. The Youngers are besieged by several issues. The collisions between tradition and modernity is brought out really well in the character of Beneatha and her relationship to Mama and Ruth. Walter is depicted as one who seeks to overcome the convergent challenges of race and class. Both of them intertwine in their attempts to beat him down, render him null and void of dreams, voice, and hope. Hansberry shows how one factor cannot necessarily be pulled out from the other. At the same time, there is much in the way of questioning how young people born into such a time of turmoil will fare. Travis' trapping of the rat might be more significant in that without proper guidance and direction, so many youth might be languishing in conditions that mirror that of the trapped rat. Finding lines or evidence from the text to help substantiate these perceptions would help to confirm that Hansberry does much well in this play.