Most people I know believe in one God or none. What is this verse actually saying?
Fair point. If you grew up around one-God religions, "Hashem is one" can sound like old news. The commentators did not read it as old news, and they do not all read it the same way.
The Sifrei (an early collection of teachings on Devarim) splits the verse in time: Hashem is our God in this world, and one in the world to come. Rashi (the classic commentator, France, died 1105) follows it. Hashem, who is our God now and not the God of the other nations, will one day be the one God of all. He points to two prophets: in Tzefaniah the peoples will all call on Hashem's name, and in Zechariah, "on that day Hashem will be one and His name one."
The Rashbam (Rashi's grandson) reads it as loyalty. Hashem alone is our God, with no other god beside Him. Of "Hashem is one," he says: Him alone we will serve. Ibn Ezra (a Spanish commentator of the 1100s) also says that "one" here means "alone." For both, the verse is about God being the only one.
The Rambam (Maimonides, 1100s) asks what kind of "one" God could be. Not one like a category with many members, and not one like a single object that can be split into parts. There is no oneness in the world like this one. He counts knowing this as a mitzvah, and he bases it on this verse.
Even the way the verse is said carries a reading. The Rema (Rabbi Moshe Isserles, Krakow, 1500s, whose notes are printed in the Shulchan Aruch) quotes an earlier source that says to pause after "Israel" and after "our God," so it is heard as: Hear, Israel, that Hashem, who is our God, is the one Hashem.
So the verse holds at least three claims: a hope for everyone, loyalty from Israel, and a statement about what God is. Each commentator stresses one. Read together, they leave room for all three.
Sifrei Devarim 31:10,Rashi on Devarim 6:4,Rashbam and Ibn Ezra on Devarim 6:4,Rambam, Laws of the Foundations of the Torah 1:7,Shulchan Aruch, Orach Chaim 61:14How can the Torah command love when nobody can make themselves feel it?
It is a fair question, and the early sources ask it too. The Sifrei (an early collection of teachings on Devarim, about 1,800 years old) says: I do not know how one loves God. Then it answers from the next verse: "These words that I command you today shall be on your heart." Put these words on your heart, and through them you come to know the One who spoke and the world came to be, and you hold on to His ways.
The Rambam (Maimonides, 1100s) says it in his own words. You can love God only through what you know of Him. A little knowledge brings a little love, and a lot brings a lot. So love is not switched on. It is built, and it begins as small as what you actually understand. He is not casual about the top of the scale, though. In the same place he says full love means being absorbed in it all the time.
Then what is the "all"? The Mishnah reads the three phrases one by one. "All your heart" means both of your inclinations, the one that pulls toward good and the one that pulls toward wrong. "All your soul" means even if God takes your soul. "All your might" means all your money, or in another reading, every measure of what God sends you. Thank Him for the hard parts as you do for the good.
Rabbi Eliezer asks why the verse needs both soul and might. His answer is that people differ. Some hold their body dearer than their money, and some hold their money dearer than their body. The verse covers whichever one you would hate to give up.
The Sifrei adds another reading of "heart": a heart that is not split against God.
So the sources do not ask for a feeling on command. They describe a direction: learn, notice what you hold back, and bring that part in too. The early Shema page gives more answers to this same question.
Sifrei Devarim 33,Sifrei Devarim 32,Rambam, Laws of Repentance 10:6,Mishnah Berakhot 9:5,Berakhot 61bCovering my eyes, stretching a word, saying it loud. Isn't that just for show?
Your siddur says to say the first verse out loud, and the custom is to cover your eyes and stretch the last word. The Shulchan Aruch (the main code of Jewish law) gives the reasons. Saying it aloud is "to awaken the intention." Covering the eyes is so you do not look at something else that stops you from focusing.
But the Gemara has a warning. In Yoma it says someone reading Shema should not signal with his eyes, wink with his lips, or point with his fingers. Rabbi Elazar Chasma says that about whoever does it, the verse says, "But you did not call Me, Yaakov."
The Tur (a law code from the 1300s) says this is why some doubted the custom of moving your head as you say the verse. His father, the Rosh, answered that the Gemara is talking about signals for something else, which ruin your focus. Here the movement is for focus and helps bring it. The Tur adds: anything a person does to wake up his focus is good.
There are limits too. The Gemara tells of a sage who told someone stretching echad a long time that he had done enough, and the Shulchan Aruch says not to stretch it longer than the time it takes to think that God alone rules in all four directions.
The test in these sources is purpose. If a movement helps you mean the words, it is welcome. If it distracts or drags on, the Gemara itself says to stop.
Shulchan Aruch, Orach Chaim 61:4-6,Yoma 19b,Tur, Orach Chaim 61,Berakhot 13b