A musical mode, or scale, is a consistent pattern of tones going up or down.
In American/Western music, we’re most familiar with two scales: the “major” scale (e.g. Wagner’s “Wedding March”), said to convey “happiness, etc.; and the “minor” scale (e.g. Chopin’s “Funeral March”), said to convey “sadness.”
The association of a scale with any particular emotion is culturally defined. In Jewish music (in Indian and Arabic music, too), the “minor” scale can be used joyously; for example, in “Hava Nagilah,” or in many of Rabbi Shlomo Carlebach’s songs. One has to wonder whether there wasn’t something deliberate in Hasidim using a musical mode meant to convey “sadness,” to instead convey “joy”; “changing sadness into joy” is a dominant Hasidic theme, isn’t it? [**] In the same sense, the scale used for haftarahs is essentially “minor,” but not “sad.” Rather, its “minor-ness” distinguishes the text from that of Torah.
Any number of melodies can be contructed using the same scale. In Indian classical music, the scale is called the “raga;” the “raga” is the musical mode, not the melody. Middle-Eastern classical music, called Maqamat, is built on the same structure: prescribed scales, used at specific times (e.g. mornings), from which innumerable melodies are composed
The Jewish musical system (“nusach”), in which a “scale” is called a “steiger” (“shtai-ger”) among Ashkenazim, includes several additional scales (as do Indian and Middle-Eastern classical music), beyond “major” and “minor.” Biblical books are publicly read with various scales, depending on the particular book and, in some cases (e.g. the High Holidays in Ashkenazic synagogues), the holiday on which the book is being read. One simple reason to vary the scales is to accentuate the difference between Torah- and haftarah- readings, or readings from other scrolls (e.g. “Esther”). Varying the scales also provides some diversity in what’s being heard, thereby helping to maintain interest in and attention on the reading.
“Among the Oriental [e.g. Moroccan] and Sephardic Jews, [the book of] Job is read on [Tisha b’Av], immediately after [the book of] Lamentations. Therefore, among these groups its mode [musical scale] has been preserved, while the Ashkenazim – not having this custom [of reading Job] – have forgotten the tradition of this mode as the “Job mode.” Instead, [the Ashkenazim] employ it for the reading of the [Torah] on the High Holidays…while all other groups read [Torah] in the same mode as is used during the year.” 
Idelsohn describes this mode as a “tune which expresses complaint and sadness,”  because of other readings with which it’s associated (in particular, the Yom Kippur reading of the death of Nadav and Avihu).
An interesting drash could probably be done, connecting the themes of “Job” and Rosh HaShanah.
The “tune” of which Idelsohn speaks means the “steiger” or musical scale with which Torah is being read. He says that this steiger “expresses complaint and sadness,” but as mentioned above, the association of a scale with a particular “feeling” varies from culture to culture.
When I’ve done High Holiday Torah-readings using the ‘Job’ scale, I haven’t at all felt that it conveyed “complaint and sadness.” It certainly seemed more subdued than the “Torah-trop” that I’d use on a “regular” Shabbat, which can be quite affirmative; even exultant. There’s an unmistakable joy underlying the holidays. If anything, the musical scales seem to temper our otherwise joyful feelings with the more serious aspects of the holidays – especially of the High Holidays – but without negating the joy.
The High Holiday Torah-mode is also very different in character from the “steiger” used for singing the prayer “U’n’taneh Tokef” – especially the verse, “On Rosh HaShanah [our fate is] inscribed; on Yom Kippur it’s sealed.” There, the nusach (both the mode, and the rhythm with which it’s intoned) seems to be intentionally somber, forcing us to take seriously that we’re being “judged,” that our actions produce inevitable consequences, and that “repentance” can be crucial.
So, as you prepare to attend High Holiday services this year, ask yourself: What feelings do the various musical elements stir up in you? How does that help your observance of the Holiday? How does it help you to resolve to make spiritual progress in your life?
(The High Holiday Torah reading can be heard on www.virtualcantor.com, as well as other online sites.)
 Idelsohn, A.Z.; Jewish Music and Its Historical Development; p. 57
[**] on Hava Nagilah as a Hasidic song:
[A.Z.] Idelsohn…composed the song Hava Nagila (Come, Let’s Rejoice), a setting of his own text, to a melody that he adapted from a Hasidic melody.
“… [Idelsohn] visited a group of Sadigura Hasidim [in Jerusalem], in 1915, and wrote down some of their nigunim. This was one of them. It was a wordless “bim-bom” melody, a mystic chant.”