The Gemara here cites the dispute between R’ Yehoshua and R’ Eliezer as to whether the episode in which Beis Shammai overwhelmed Beis Hillel and issued eighteen new decrees was a good thing or a bad thing (see the description of the even earlier in the mesechta, 17b).
Hillel, the first of the Sages of the Mishnah, was eulogized as a student of Ezra HaSofer (Sanhedrin 11a). Indeed, both Sages ascended from Bavel to Eretz Yisrael and renewed interest and involvement in Torah.
R’ Tzadok HaKohen of Lublin (Takanas HaShavin #5) notes that it would seem that they were of very different bents. Ezra and the Anshei Knesses HaGedolah were very much involved in instituting seyagim and takkanos - boundary prohibitions surrounding Torah law, and enactments meant to promote greater observance (see the beginning of Pirkei Avos), while Hillel and his students opposed the institution of newer and greater strictures in Halachah (see Tosafos to 14b d.h. V’Ilu and here d.h. Bo Bayom. This was in line with Hillel’s general trend towards leniency. How, then was Hillel a student of Ezra?
Reb Tzadok resolves the difficulty. He notes that, clearly, Hillel was not literally a student of Ezra, who lived many years before him. Rather, he followed in Ezra’s footsteps in that he was humble and pious (as the Gemara in Sanhedrin frames the eulogy: הי עניו הי חסיד ). However, in the generation of Ezra, humility and piety required more seyagim and takkanos. This was essential for the success and maintenance of that generation’s teshuvah, which was not complete. Since they did not achieve full teshuvah, there prior sins were not reversed, and new enactments had to be introduced in order to prevent backsliding.
In Hillel’s time, however, the great love he displayed towards others (see Pirkei Avos 1:12) influenced them to turn with love towards Hashem. Teshuvah done out of love is complete teshivah. In such circumstances, Chazal tell us that the sinner’s pasts transgressions metamorphose and become merits. Hence, those sins would not lead to backsliding - as they themselves have been changed into mitzvos. Therefore, new enactments and boundaries were not necessary.