Like the majority of the posts above, I don't think we should see this play as a tragedy. Perhaps there are tragic elements, but these can also be understood as simply being socially entrenched conflicts.
One strong argument against this play being classified as a tragedy is that the characters are not flawed in themselves, but are subject to pressures put upon them by circumstance - and they each conquer those negative forces in the end.
I have to agree--this play isn't a tragedy. There are certainly some tragic elements, such as losing some of the investment money, an "almost" abortion, a young black man who's so angry and frustrated at the world, feeling as if everyone is against him. And more. However, I take heart that things will be better for the Youngers for a couple of reasons. First, there will be a new life born into the family rather than a tragic death. Second, this opportunity has renewed their energy and enthusiasm, and working for something they own will be less tedious than working on a treadmill, getting nowhere. Third, they'll be out of that dismal and overcrowded apartment. I know they're heading into a racially charged environment, and I'm not implying they may not have some trouble; however, the last thing Mama grabs as she's living their old place is that plant, symbol of hope. It's the one thing which continued to grow in that environment, and I have hope it will continue to do so in their new environment.
I have to disagree with the question based on this play. Of course, A Raisin in the Sun contains many tragic themes, however, the resounding tone above all is an optimistic one - in spite of the many pressures that the Younger family face and the disasters that they undergo such as Walter's loss of the money through his financial imprudence, by the end of the play the Younger family have resisted these internal and external pressures that threaten to divide them and remain united, and we as an audience are left confident that they will continue to brave the challenges that lie ahead of them as they have braved the challenges in the play.
"A Raisin in the Sun" could be argued a tragedy because of how much was lost to the Younger family because of racism. Walter is so jaded by the way people treat him and his family that it leads him to destroy the most important relationships in his life. For example, he starts drinking very heavily to deal with the rejection he feels Mama puts him through by not believing in his dream to start a liquor store. Walter turns his back on Ruth when he finds out she is pregnant by not acknowledging what could be a happy event. When Walter is given Mama's trust, he loses his father's insurance money by trusting strangers over his mother's warnings.
The title comes from the poem by Langston Hughes "A Dream Deferred" and the message there is that a dream that never comes true can destroy someone. Walter and his family are constantly putting their dreams to the side. Even when they buy a home, they are disappointed by the racism that begins even before they get there. So although most would argue that the family overcomes all these problems in the end, most of the story is one of heartbreak and tragedy that the Youngers must continue to overcome.