Who was "one of the youths"?
Rashi: It was Do'eg ha'Edomi.
What is "Yodea Nagen"?
Sanhedrin 93b: He knows how to ask.
Malbim: He has full knowledge of the Chachmah of music.
What is "Gibor Chayil`"?
Sanhedrin 93b: He knows how to answer.
Radak: He has Gevurah (courage) of the heart for everything, like we find that he struck the lion and the bear.
Malbim: This is unlike most singers, who have a weak nature.
What is "v'Ish Milchamah"?
Sanhedrin 93b: He can debate in Torah.
Radak: He knows matters of war, and he is used to it.
Malbim: He knows war strategy. Usually, such a person does not engage in music and song.
What is "Navon Davar"?
Sanhedrin 93b: He understands a matter through another matter.
Radak citing Targum Yonasan: He has wise counsel. For everything his counsel is correct, like "v'Navon Lachash" (Yeshayah 3:3).
Malbim: Usually, people of real intellect and [good] counsel are not singers.
What is "v'Ish To'ar"?
Sanhedrin 93b: He brings proof for what he says.
Radak citing Targum Yonasan: His form is nice. Similarly, "Anshei Midos" (Bamidbar 13:32) are [giants with] large measures.
Malbim: Most singers are dark and do not have a nice form.
What is "va'Shem Imo"?
Sanhedrin 93b: The Halachah always follows him.
Radak: He succeeds in everything.
Malbim: He fears Hashem and evades evil. It is rare that a beautiful singer is not drawn after his lusts and desire for women.
Why did he speak so highly of David?
Rashi (from Sanhedrin 93b): Do'eg said all this to arouse Sha'ul's envy of David.
Radak: The last five are needed for anyone who will stand in front of the king. Knowledge of music was needed in this particular case.
Malbim: Since David was a youth, he praised him to say that he has all the proper natures.