The torah documents the positive attributes of Sarah, Rifka and Rochel. Chazal then expound in greater detail about these righteous women. But from the torah and chazal there doesn't appear to be any material indicating any specially positive attributes about Leah. What was so special about Leah that she merited to marry Yaakov and be considered a matriarch of klall yisroel?
Well for starters it is mentioned that her eyes were "Rakos" since she cried a lot as she did not want to end up with Eisav. This shows that she appreciated only good.
In addition she gave up having another boy - Rashi Breishis 30:21 - in order for her sister not to feel as the one with the least boys.
In addition to Gershon Gold's answer, there's also Bereshit 30:17:
וישמע אלהים אל לאה
and Rashi says there:
שהיתה מתאוה ומחזרת להרבות שבטים
which is also a good thing.
Oh, and an even stronger makor, Leah was also a prophet, as said in Rashi on Bereshit 29:34 :
ותהר עוד ותלד בן ותאמר עתה הפעם ילוה אישי אלי כי ילדתי לו שלשה בנים על כן קרא שמו לוי
לפי שהאמהות נביאות היו ויודעות שי"ב שבטים יוצאים מיעקב וארבע נשים ישא אמרה מעתה אין לו פתחון פה עלי שהרי נטלתי כל חלקי בבנים
We know why she married yaakov- lavan tricked him. What makes the avos/ imahos who they were vs 12 tribes? Think process of selection;; rejection in genesis