1) Rambam has his own mysticism, a naturalistic one: How does one come to love God? By contemplating the grandeur of Creation in all its details. By studying nature, in all its logic, order, system, symmetry, one encounters the Infinite Will that created all of it. The psalms about creation praising God allude to this idea (Ps 148:1-6).
2) Think about Adon Olam - which some use to close the prayers of the day. Master of the Universe who reigned before anything physical was created. His reign was absolute without man or fruit trees to defy His Will. So at the moment when God created the Sun and Moon, His Kingship was absolute. By celebrating the return to that physical moment, the position where the Sun was at the moment of Creation, we celebrate God's Kingship. Psalm 19 also alludes to this, in suggesting that the Sun has volition, deciding to jump out every morning. Before the Sun exercised volition, then, God's Will was the only Will.
3) Hallel Hagadol is also commonly said. The refrain: ki le'olam chasdo, that His chesed is eternal, tells us that the creation of the Great Luminaries was because of His Chesed. And indeed, the great luminaries supply Earth and all life with the energy they need to exist, just as God directly gives us existence-flow. Avraham saw the Sun and realized that it was an instrument of God's Chesed, which resonated with his own attribute of Chesed. So this verse brings us to a re-enactment of Avraham's journey of discovery of monotheism.
4) Why Nisan not Tishrei? Tishrei is the creation of the physical world, while Nisan is the root of Jewish history. Well, if you look at the flow of Jewish history, the Sun is often the instrument of Divine aid to the Jews. The sun rushed down (Gen 28:11) so that Jacob would stop davka where he would dream of the ladder, and then those two hours were given back during the fight with the angel. The sun stopped for Joshua. The sun stopped to make a point that Nevuchadnetzar would see as far as away as Bavel. The sun is harnessed to Jewish history - though we are a small people, we have a great Protector. Many verses in the liturgy remind us of this.
All of these points were reinforced with Talmudic and midrashic aggadita, taken metaphorically as intended - since we have trouble today accepting ideas such as the Harmony of the Spheres literally, we still see harmony and grace in physical law. He closed with some personal observations, for which you should listen to the tape (probably available soon on their web-store), and a wish that next cycle if not this cycle should be observed not only in the proper time, but in the proper place, the place of Creation, Jerusalem at the Temple.
Torah in Motion, the organization that put on this shiur, has two more great-sounding shiurim on Pesach and the Haggadah coming up this evening. Do sign up for them, and for their ongoing shiurim. I've bought tapes of their occasional special-interest conferences, which bring great speakers together to talk about fascinating topics: Tanach, Jewish community interests, historical personages, etc.