The Shulchan Arukh discusses in three places the qualities preferred in a cantor:
1. In Orach Chaim 53 (4-14), the requirements are presented for being appointed a permanent cantor. They are: free of sin, humble, desired by the people, has a good voice, learned in the Bible, and able to grow a beard. Interestingly, the Rema (5) writes that a 13 year old with a bad voice who understands the prayers has priority over an ignorant old man with a good voice.
2. In Orach Chaim 579 (1), the Shulchan Arukh lists the requirements for leading prayer on a fast day.
3. And in Orach Chaim 581 (1), the Rema lists the requirements for being selected as the leader of selichos.
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However, the Shulchan Aruch (O.C. 53:11) makes an important limiting remark based on a responsum of the Rashba: A cantor who lengthens the prayers in order to show off is repugnant and wastes the congregation's time. This is in contrast to one who lengthens the prayers out of joy for praising God, who is acting properly. Evidently, cantors showing off is a long-standing problem because we find it discussed throughout centuries of halakhic literature.
The Maharshal (Yam Shel Shlomo, Chullin 1:50) complains about this in particular on Yom Tov (cf. Magen Avraham 529:intro). The Bekhor Shor (Ta'anis 16b) writes at length about this issue, sharply condemning cantors who act in this way and writing that it is better to go to a theater to hear a concert than to desecrate a synagogue with one. He even quotes a poem written by his brother against cantors who show off. The Chayei Adam (138:4) also gets in some cantor bashing. It seems that no posek can let this matter pass without commenting negatively on cantors who show off.
Interestingly, the Arukh Ha-Shulchan (O.C. 53:13) writes: "There is much to say about cantors in our time but just like it is a mitzvah to say [that which is heard, it is also a mitzvah not to say that which is not heard.]" I think that by his time, the literature had already said it all about the public servant everyone loves to hate.