Beating my first dragon in Baldur's Gate 2, and becoming much stronger (a true dragon killer) in the process. I recall my paladin obtained a kickass sword from that fight that he probably used for the rest of the game. I got lucky with that fight though, the dragon teleported to a small room with me and couldn't move there, and I was able to kill it from a distance. But who cares, a win is a win!
Finding out that some seemingly meh game has kickass music. This occurred to me when seeing the Shovel Knight trailer. Puzzle platformers or whatever it is aren't really my main interest, but I'll probably buy the game at some point anyway for the music.
https://www.youtube.com/watch?v=MHLgqTEQYX8 A bit similar was how in Wing Commander Prophecy I found the flight (mission) music quite boring, but when I played the standalone expansion pack (Secret Ops), it had great pumping music during missions (it is the same music as in the simulator missions in Prophecy).