For a hentai go look at 16.5. For fan service you must be a blind man not to notice it. As for fillers the thing with that girl and her gang that got killed off in a trap by a bunch of dwarfs was never of any use. Same goes for that guy who killed his wife. Also there is no development for the main characters since Asuna went from being a badass female lead to a generic incapable tsundere. More like character downgrading.
I added the Asuna part in the edit.
There is legitimately no fanservice/barely any in this anime.
And there isn't a season 2 either, you guys keep saying season 2 but that's Gun Gale Online.
ALFheim Online could've been a season 2 but they butchered it, it's way shorter than the volumes in the novel.
The rest here is spoilers for anyone else who is reading.
The girl and her gang who got surprise attacked did serve a purpose through the entire series if you haven't noticed, it was all part of Kirito's development.
The guy who killed his wife did have a purpose, although it wasn't as significant as the other side stories.
It involved Kirito and Asuna's development, not to mention the story of Kirito sleeping under the tree and Asuna coming along, that was a major part actually, it has been recalled numerous times in the novel.
The explanation of marriage between people was also quite significant.
Explanation about armor breaking and teleporting too.
The rest of it was all not needed though.
Asuna isn't really a tsundere, she actually liked Kirito from the start, the tsun part was just a facade.
And if you tell me to look at 16.5 you're telling me that there is fanservice in 1 episode?
I haven't even watched that episode because it's most likely a recap if it's .5