Oh… Baruch Hashem…. my memory seems to be returning. Mixolydian, if I recall, is a mode of sound which uses the same tones as the major scales, except for the seventh. How could I ever have forgotten? 🙂
Bottom line: Neilah is in a major scale, whereas Tal/Geshem are in minor.
By the way…. talking about nusach…. what’s most frustrating are the errors people make in the nusach of Kaddish.