B'rachos, 54a: It teaches us the obligarion to recite a B'rachah over a miracle. ¹

Sanhedrin, 94a: It is an embarrassment that Yisrael did not bless Hashem until Yisro came and and said "Baruch Hashem". ²

Ramban: It refers to Moshe and Yisrael.

Rashbam and Seforno: It refers to Moshe and Aharon.

Oznayim la'Torah: It refers to Moshe, Aharon and the elders who accompanied Moshe and Aharon to greet Yisro.

Rashi: He was referring to the fact that, not only did Hashem save Yisrael from Egypt (a tough nation) and from Par'oh (a tough king), but He also rescued them from a particularly harsh subjugation to which they were subjected.

Ramban #1: He was referring to two independent miracles - 1. to saving Yisrael from the hands of the angry Egyptians (after each plague), and Moshe from the clutches of Par'oh whenever he came to warn him; ¹ 2. to the rescue of Yisrael from the subjugation of Egypt and taking them out to freedom.

Ramban #2 (citing the Ibn Ezra), Seforno and Rashabm: Refer to 18:10:2:1, only he said "Eschem" with reference to Moshe and Aharon, whom Hashem rescued from the clutches of the Egyptians of Par'oh.

Oznayim la'Torah: Refer to 18:10:1:3. Yosro said "mi'Yad Mitzrayim u'mi'Yad Par'oh" with reference to the leadership of such cruel masters, which the elite of Yisrael resented; whereas "mi'Tachas Yad Mitzrayim" he said with reference to the harsh conditions of the slavery, ² which all the people suffered - which Yisro now blessed Hashem for rescuing them.

Oznayim la'Torah: He was afraid that, since Yehoshua had only weakened Amalek, and not destroyed them completely (as opposed to Mitzrayim, whose every soldier was drowned in the Yam-Suf), If they would hear that he was happy about their downfall, they would take revenge. ¹