Harmony is indeed an interesting thing. Two aspects of it that I think are most notabe:
1) There are levels of complexity of harmony, and discord is merely the point at which the complexity exceeds our ability to appreciate it. Furthermore, this point varies for each individual.
2) I think that you can have harmony in consiousness, and also harmony in the subconscious. For example in a display face harmony can be promoted through high formal modularity and alignment; but the way a text face is used might make these harmonic tactics actually backfire. You could say text faces need a deeper harmony to really work, but when set at a large point size it looks like discord.

I would recommend a wonderful little booklet called "The Harmonograph" by Anthony Ashton (Wooden Books Ltd, 1999). Although the book is essentially about a peculiar undulating device (popular in the Victorian period) which draws spirals and such of varying complexity, it does contain some great nuggets of harmonic wisdom - such as this: "Music with only simple harmonies can seem naive and uninteresting. The more complex harmonies are intriguing, challenging."

hhp

Too much harmony/unity/symmetry ... Too much discord/incongruity/assymetry ... Both can be beautiful and ugly. The wonderful thing about all things creative manifested in objects of all varieties is that it is all subjective. Of course there are things that some will agree upon, but those same things can become a rift between two designers that were friends. Some typefaces are beautiful in an of themselves. But those same typefaces don't guarantee a well thought out typographic scheme. For everything there is an opposite, and that is what makes some beautiful, I think so anyway.