Leah named her first three sons for how she perceived her relationship with her husband, Yaakov. But when it came to her fourth son, she named him Yehudah, expalining, "hapa'am odeh es Hashem" this time I will thank G-d. This is considered a tremendous thing by Chazal. In Brachos 7b it record that R' Yochanan said in the name of Rav Shimon bar Yochai that from the day that Hakodesh Baruch Hu created his world there was not a single person who thanked Him until Leah did with "hapa'am odeh es Hashem."
My granfather points out that on the same page of the Gemara Rav shimon ben Yochai is quoted as saying that from the day that that Hakodesh Baruch Hu created his world there was not a single person that called Him that Hakodesh Baruch Hu created his world there was not a single person [master] until Avraham came and called Him Adon.
The commentaries explain that until Avraham came along, people said that Hashem left His world after he created it. Like the Daoists, they believed that he set everything up to operate on its own according to the laws of nature but that He doesn't relate to it directly. Avraham's use of the name Adon made it know that not only did he create the world but that He is also its Master who is aware of and responsive to what goes on in it.
According to this, we have to understand what Leah added by acknowledging her gratitude. Isn't it implied in the term Adon that we acknowledge that we receive all from the Maste of the world? That was already established from the days of Avraham Avinu.
My grandfather suggests that what she added was the gratitude that we should express after receiving, pointing out that Hashem is the one who gives, and that's the idea of such statements of appreciation.
Human nature doesn't always see it that way, as he explains with reference to an explicaton of a passage in Brachos 35b. R' Levi Rami said, it is wrtiten "la'Hashem har'aretz umloah" [to Hashem is the land and all that fills it] But it also is written "Hashamayin, shamayimn laHashem, veharetz nathan livney adam" [The heavens above are for Hashem, and the land He gave to the sons of man] He said it's not a contradiction; the first part refers [things like food we eat] before the blessing, and the second to the situation after the blessing.
Human nature, though, can suggest another interpretation. Before the bracha, that is before a person get what he wants, knowing that the land and all within it belongs to Hash, one prays to receive. However, after that bracha, that is after receiving what one wants, a person says in his heart, veharetz nathan livney adam, thinking kochi veotzem yadi asa li et hachayil hazeh.
In other words, what Leah did was counteract that tendency to forget about acknowledging Hashem's role when things go the way we want them to.