What is "Efod Bad"?
Rashi citing Targum Yonasan: Kardut d'Vutz (a linen robe). Kardut is a robe, for so he translated "Tilbashnah ha'Besulos Me'ilim" (2:13:18).
Radak: Yonason translated Me'il Katan (in verse 19) simply (a small robe). It seems that there are different kinds of Me'ilim. Shmuel and the Kohen Gadol wore a robe to wrap [the body] - "v'Hu Oteh Me'il" (28:14). The Me'il of Benos ha'Melech was made like a woman's garment. Also there were two kinds of Efod. The Torah specified how to make the Kohen Gadol's Efod (Shemos 28:6-8). Those who serve Hashem used to wear a linen Efod, to distinguish them from others. This applied to Kohanim, Leviyim and Yisraelim. We find 85 Kohanim "Nosei Efod Bad" (22:18), and Shmuel (a Levi) wore one, and also David (a Yisrael) - v'David Chagur Efod Bad" (2:6:14).
Verse 11 already taught that he served Hashem!
Radak: After teaching the detriment of Bnei Eli, it teaches the merit of Shmuel, that he did not learn from them to engage in eating and drinking and worldly pleasures. Rather, he served Pnei Hashem, to learn Torah, know Hashem, learn Avodas ha'Leviyim, i.e. verbal and instrumental Shir.
Malbim: Even though Bnei Eli were Kohanim, Bnei Kohen Gadol, and older, and Shmuel was not, even so, he served Hashem (unlike them).
Why does it say only that he served Hashem? Verse 11 said that he served also "Pnei Eli"!
Malbim: He already began to recognize Elokus by himself.
