To this day, my favorite thing about Sword Art Online is the main characters' romance arc. Everyone always focuses on Kirito being overpowered, like that's not a trope in 75% of anime from the past decade. What I love is that they actually had Kirito and Asuna start dating early in the series and stay together casually showing their relationship and letting it grow over time.
I'm so tired of 99% of anime, even literal romance anime, making you wait until the last few episodes (if at all) for the characters to finally get together, after teasing it for the entire series. And when they do, it's barely realistic or satisfying. (I know other ones here and there also do it but Ive watched nearing a 1000 anime and 99% dont)
I obviously love other aspects of SAO too, but the thing I appreciate most is how they handled Kirito and Asuna's relationship. It actually makes you connect to them and their relationship instead of doing the usual "will they, won't they" edgy dragged nonsense, constantly finding random plot devices or unrealistic reasons or even having them do out of character stuff to keep the protagonists from dating like 99% of anime do. It's so annoying to me, I don't know if mangeka can't or are just afraid to write actual couples but I'm so tired of an entire series of forced build for a episode if that of them actually getting together. This is something SAO did extremely right in my opinion especially being a power fantasy. (Tho I do dislike that in following seasons to an extent they kinda tossed Asuna aside for the majority of the run time)