Is a yibum marriage a full marriage? For leaving it, it would seem so - requiring divorce and not chalitzah. And the woman who is now divorced from the yavam is treated as a divorced woman in regular cases. Also, a new mishnah that could pass for the tractate's opening mishnah - about how yibum works, who is obligated, etc. For example, one who is obligated by the yavam and tries to pan it off on someone else...(specifically, his brothers). Plus, how specific amoraim connected to each of the people who encountered yibum, with actual practical cases. And some open questions on the structure of this masekhet.