"Vayichad Yisro al kol hatova" -- Rashi explains that Yisro got goosebumps because he felt the pain of the Egyptians who had been killed, or, according to another interpretation Rashi offers, he was filled with joy over the news of Klal Yisrael's deliverance. Yisro then exclaimed, "Baruch Hashem asher hitzil eschem!"
Chazal read this pasuk as an implicit criticism of Klal Yisrael because Yisro was the first to say "Baruch Hashem" and they were not (Sanhedrin 94). It's hard to understand what bothered Chazal. True, no one said from Klal Yisrael had said those exact words, "Baruch Hashem," but Klal Yisrael sang shirah -- isn't that enough thanks and enough praise? Is there something magic about Yisro's words that made them more significant than the entire shiras ha'yam?
Ksav Sofer offers two answers, each of which is a worthwhile limud in its own right:
1. Geirus does not psychologically divorce one from past life history -- according to the first view in Rashi, Yisro empathized with the Egyptians and felt pain over their death. Bnei Yisrael had no such qualms or regrets. Of course it is wonderful to be able to sing shirah to Hashem when you are elated, like Bnei Yisrael at Yam Suf. But its even more wonderful if you can sing shirah or say "Baruch Hashem" like Yisro, even when the good experienced produces mixed emotions or comes at a cost. Bnei Yisrael are criticized because although they sang shirah over their redemption, they should have also given thanks for the shibud itself, as tragic and painful as it may have seemed.
2. Yisro fled Egypt and never experienced avdus first hand. He did not need mon to eat, he did not need to do battle with Amalek. To sing shirah over the tremendous miracles one experiences in times of need is wonderful. To thank Hashem for the miracles done for other people -- to share in their joy and feel happiness for their success -- that's exceptional. Bnei Yisrael are criticized because each person sang shirah for his/her personal redemption, but did not think to sing shirah over the redemption of his fellow Jew as well.
I think the Brisker Rav was troubled by this same issue and wanted to make the same point (in a way that is rooted in halacha, k'darko) as this second answer as the Ksav Sofer.
The Brisker Rav explains that there is precise link between "Vayichad Yisro" and Yisro's "Baruch Hashem." The Shulchan Aruch (O.C. 219) paskens that Reuvain is allowed to say a birchas hagomel for his friend Shimon. It's not a bracha l'vatalah, explains the Rama, because a birchas hagomel is nothing more than a statement of thanks to Hashem for deliverance from danger. If Reuvain truly feels personal joy over Shimon's deliverance (see Taz, quoted in M.B.), e.g. Shimon is a close relative or friend, then Reuvain has ever right to say a bracha of shevach to celebrate. Since Yisro felt such overwhelming joy for the sake of Klal Yisrael, "Vayichad Yisro," therefore, says the Brisker Rav, he had a right to say a bracha, "Baruch Hashem..." over their salvation.
Rav Sorotzkin in his sefer Rinas Yitzchak is troubled by why the Brisker Rav needed to give us an explanation to justify Yisro saying a bracha when the gemara itself does so. The gemara (Brachos 54) explains that Yisro's "Baruch Hashem" is the source for the din that one is obligated to say a bracha when one sees a place that miracles occurred to Klal Yisrael. (Yisro saw the mon, etc.). Yisro's bracha was a birchas hanes, not a birchas hagomel.
I think perhaps the Brisker Rav's concern was not the halachic nature of the bracha per se, but rather what made the bracha so special. Birchas hagomel provides a halachic model for this type of personal joy that results from seeing the success or deliverance of others.