I was asked to comment on adviken, an interesting guy that posts music theory lessons on YouTube. His lessons are actually quite interesting in the sense that he can explain some things quite well, like the tonal relations and scale variations. However, there isn't much more there.
There are a couple of things that I've heard some time ago that intrigued me in relation to music theory:
1) Our ears are not as relative as we once thought. Classic music theory claims that if you play a piece in C major or C# major you won't feel the difference. And that's actually not true. Actually it was something that a lot of composers already claimed not to be true - some gave colors to different tonalities like Scriabin. Also I'm not falling for the statistics discussions that claims that there are more pieces in C, F, Bb and G major than any other key as proof for key preference, because that is probably mostly related to the difficulty of playing specific keys in some instruments. There are some interesting experiments that showed that people can sing their favorite song in the correct key only using their memory. Quite interesting
2) Sound compositions is extremely important. You learn that very quickly the first time you try to compose something with more than 4-5 instruments. It makes a huge difference to forget a little about harmony and counterpoint theory and just think about what you can do with two or more instruments that will sound interesting. Percussion is great for it. It has been proven that the attack of an instrument is what gives most of the difference in the sound. If you add a percussion to the attack, you suddenly have something very strong messing with the instrument identifier and what do you get? A "new instrument"!
3) Harmony and consonance is mostly cultural. In different cultures people get to (approximately) different harmonies and your ear gets used differently to what "soundg good". And that begs for the question: now that we are continuously moving towards a globalized world, are we losing this richness in hearing? Are we drifting towards somehing else?
Music is exciting! But sometimes work is necessary.