Why was Hashem so pleased with Yisrael's words?
Rashi (in Avodah Zarah, 4b): Because 'they were strong and powerful in their fear of G-d'.
Ramban (citing B'rachos, 33b): Because 'Everything is in the Hands of Hashem with the sole exception of Yir'as Shamayim', 1 in which case their fear of Hashem was exacly what He wanted from them.
Refer to 5:26:2:1.
Ramban: Yir'as Shamayim is in the hands of the people, and Hashem was expressing the hope that Yisrael would retain the level of Yir'as Shamayim that they had reached then (like a father wants only what is good for his son
What are the implications of "Mi Yiten Vehayah Levavam Zeh La'hem ... "?
Avodah Zarah, 4b: It implies an opening for Ba'alei Teshuvah, in that their Teshuvah will be accepted, just like the Teshuvsh of Yisrael was accepted after they sinned by the Egel. 1
Avodah Zarah, 5a: Moshe was accusing them of being ungrateful - because they did not acknowledge Hashem's kindness (or did not want to thank Hahem). 2
Oznayim la'Torah: It conforms to Chazal, who say that, whereas Resha'im are controlled by their hearts, 3 the hearts of Tzadikim are controlled by them. 4 Here too, it implies that Yisrael's hearts will always be under their jurisdiction to fear Hashem. 5
See Torah Temimah, note 16.
See Torah Temimah, note 17.
Oznayim la'Torah: As the Torah writes in Toldos Bereishis, 27:41 "Vayomer Eisav be'Libo" - and similarly in connection with Yarav'am, Naval and Haman,
Oznayim la'Torah: As the Torah writes in Shmuel 1, 1:13 "Ve'HInei Chanah Medaberes al Libah" - and similarly in connection with David and Daniel.
See Oznayim la'Torah, who elaborates.
What are the implications of "Kol ha'Yamim"?
Oznayim la'Torah: And not just whilst the mountain is being held over their heads!