The gemara (Brachos 7b) writes that there was no one since creation who gave thanks to Hashem until Leah said “hapa’am eodeh es Hashem” and named her son Yehudah (29:35).
Could it be that Avraham, Yitzchak, Ya’akov never gave thanks to Hashem? Impossible to believe. Just turn back to parshas Chayei Sarah and you find that even Eliezer said “Baruch Hashem” at finding success in his search for a shidduch for Yitzchak. So whaydo Chazal single out Leah and the one was mechadesh giving thanks?
Ksav Sofer writes that the Avos lived lives where the supernatural was the norm, and they of course reciprocated with thanks and appreciation. Leah gave thanks for childbirth, something that you can see in a hospital any day of the week.
This is why, he explains, Chazal were critical of the person who says hallel daily. Hallel is a prayer of thanks for supernatural deliverance. Someone who says hallel daily is fixated on the miraculous at the expense of appreciating and giving thanks for the regular day to day.
Others explain (I saw this explained nicely in Toras HaAggadah by Ephraim Oveid, but the idea is already in the Ben Yehoyada) that what made Leah’s thanks special is the fact that she incorporated it into the name of her child. For most people, a thank you comes and goes. There may be gratitude today, but it’s forgotten and replaced by “What have you done for me lately?” Leah wanted to remember that thank you always, and she wanted that spirit of gratitude to become part of who her child was. That’s a good lesson to take to heart even if you don’t name your child Yehudah.
Rashi explains that Leah gave thanks here because in having her fourth child she realized she had gotten more than her share. She knew that Ya'akov would have 12 children, divided by 4 wives, means her share should have been only three, and now she already had a fourth child. Maharasha writes that Leah was not speaking b’toras nevuah, i.e. she did not know through prophecy that there would be 12 children in total or that none of the other wives of Ya’akov would have more than three children, as we don’t find Leah listed (Megillah 14) among the seven women nevi’os. This seems to contradict Rashi in our parsha (27:12) who writes that all the Imahos were nevi’os (the Sefas Emes brings citations from Midrashim that say the same). The Targum Yonasan also writes that Leah chose the name Yehudah because she saw David haMelech in the future, who would extol Hashem with his praises of thanks – again, another proof that Leah had prophecy and used it specifically here. The simple answer is that Midrashim need not be consistent with each other. Maharal answers that there were many other people , like Leah, who had the gift of nevuah but who are not listed by Chazal because the list consists only of those people whose prophecy is quoted in the text of Tanach.
Coming back to the question of what made Leah's thanks special, I think there are two possible ways to understand the Targum Yonasan I mentioned above. One way is that he is telling us a siman: the fact that a David haMelech, author of Tehillim, would come out of Yehudah proves that the midah of thanks and praise was in Yehudah's spiritual DNA; therefore, he deserved that name. But one could understand it as a sibas as well: Leah named her child Yehudah because she wanted to already commemorate the thanks and praise that she saw her great...great grandchild singing. If that's what the Targum means, then maybe what made Leah's thanks special is the fact that she was showing gratitude for something that would not happen until years later in the future. We have enough trouble showing gratitude for the chessed done for us in the here and now; Leah showed us that you can have gratitude even for what is yet to come. Does the fact that she was a nevi'ah and knew with a greater degree of certainty that it would happen change things? Maybe. But I still think the point stands. "Shema yigrom hacheit" -- there is always uncertainty. Leah still had to have a degree of trust and hope. Leah shows us that you don't always experience all the rewards in the here and now, but you can certainly give thanks for them.