Basic chemistry. I hated chemistry the first 2-3 of years of high school (UK; I don't know if it's taught differently elsewhere). It was all about laundry lists of chemicals, their apparently random properties, and mixing them according to haphazard instructions with results that very occasionally corresponded approximately with what we were informed they should be. We were sort of shown the periodic table, of course, but not really enlightened as to what it all meant. I found it boring and pointless. I hated memorising the properties and relationships of the chemicals we were supposed to know about.

Then, all of a sudden (I think right at the start of year 10), they told us about electron shells. There was rhyme! There was reason! There were underlying, and actually rather enthralling and beautiful, explanations! The periodic table made SO MUCH SENSE. It was too late for me... I had already pretty much solidified in my dislike of chemistry, and had decided not to take an excessive amount of science at GCSE because similar (though less obvious) things had happened in biology and physics, too. But at least I did get that small set of revelations. Why on earth they didn't explain it to us like that right from the start, I have no idea. I would have loved it.