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
There are a couple of points that are noteworthy about Leah and make her resultantly stand out:
1) Her Gratitude
One major praise of Leah is the fact that she showed her hakaras hatov (her gratitude) for when she had more than her "allotted" number of children.
The pasuk in Bereishis 29:35 writes:
וַתַּ֨הַר ע֜וֹד וַתֵּ֣לֶד בֵּ֗ן וַתֹּ֙אמֶר֙ הַפַּ֙עַם֙ אוֹדֶ֣ה אֶת־יְהוָ֔ה עַל־כֵּ֛ן קָרְאָ֥ה שְׁמ֖וֹ יְהוּדָ֑ה וַֽתַּעֲמֹ֖ד מִלֶּֽדֶת׃
She conceived again and bore a son, and declared, “This time I will praise the LORD.” Therefore she named him Judah. Then she stopped bearing.
As the midrash in Bereishis Rabbah 71:4 points out - there were four wives of Yaakov and so the thought was that they would each give birth collectively to the 12 tribes, i.e. they would each bear 3 children. So when Leah had her fourth child she realised that she was blessed beyond the number initially thought.
Indeed the Gemara in Brachos 7b highlights this level of thanks as being a first in the Torah:
And Rabbi Yocḥanan said in the name of Rabbi Shimon ben Yocḥai: From the day the Holy One, Blessed be He, created the world, no one thanked the Holy One, Blessed be He, until Leah came and thanked Him, as it is stated: “And she became pregnant and gave birth to a son, and she said, ‘This time I will give thanks to God,’
[Note the Ksav Sofer who further develops this unique demonstration of gratitude.]
2) Sensitivity to her sister
(Building on @GershonGold's second point)
In the same way Rochel is praised for not embarrassing her sister Leah by showing her the signs (see Bava Basra 123a), in a similar fashion Leah is praised for being sensitive to her sister's plight.
In the case of Leah, she realised that she had been blessed with six children and that the maidservants Bilhah and Zilpah had already given birth to two children (taking the total up to 10 shevatim). When Leah became pregnant again, she was worried that if it were a boy her sister Rochel would be slighted as she would contribute less shevatim than the maidservants. Leah accordingly davened that the child she was carrying be a girl (see Berachos 60a and Targum Yonasan 30:21 where it says that Dina and Yosef were swapped in the womb).