That interesting, since for lots of people who disliked Serena, that episode was the turning point in their feelings toward her. Many people thought XY60 was one of the best episodes in all of Pokémon for it's emotional gravitas. It's easily the moment you can point to when Serena started becoming a more popular character, especially in the western fandom who had derided her up until that point.
I think of of my favorite things about the subplot is how it was worked into Ash and Serena's individual plots. Ash's Gyms had tons of worried shots as well as Serena trying to help at times (Viola, Korrina, Wulfric) and the Tripokalons usually brought a character to tease Serena (Shauna, Millefeui) or had Ash turn into a Pokémon like Serena was his name. It wasn't obtrusive, but it reminded us that there was something being built between the characters during the series.