I remember playing the D/P slot machines a bit... then getting the Clefairy.
Excited, naďve me thought: "Yay, Explosion TM!" and followed its directions. It didn't feel like I had any sort of control of what happened, I followed the directions and won. I was one single turn away from getting the TM...
... Then suddenly, somehow, its directions were wrong and I lost. Game over.
Also, I remember raging a little when I found out that Voltorb Flip was the ONLY way to earn coins in HGSS. The most useful TMs ever were also insanely expensive, it took an eternity to get to afford one of them (where were I supposed to get 10,000 coins from? I had the money, dammit!). In comparison, FR/LG practically handed them out for free.
Oh, and as mentioned... the music. It's been awesome in every generation I can remember it from.