The Yagami family as a whole is very popular among the fandom. Hayate being a fan-favorite, and the Wolkenritter being very popular and the Unison Devices being also very liked, it's not surprising why there are so many fanart about the them.
Evil Is Sexy: Reinforce, for a given value of "evil". Also, the last boss has breasts for no apparent reason other than this.
Growing the Beard: For the entire franchise as a whole, while the original show certainly had a lot of memorable moments the series was still coming into it's own and had a lot of teething problems. A's is considered the point that the Nanoha franchise truly hit it's stride with the series mixing in the best of both worlds from it's Magical Girl beginnings to its Super Robot / Space Opera roots, keeping the majority of the supporting cast from the first season around as well as introducing new favourites and the seasons intriguing Grey and Grey Morality. The season is so well received that every entry in the franchise from this point forward has had to follow in this seasons shadow.
Jerkass Woobie: Vita, before meeting Hayate, was a jerk even to the other Wolkenritter as shown in the sound stages, and harshly rebuffed Reinforce's efforts to reach out to her. She who also the one who seemed to suffer the most from a life of constant fighting.
Either you loved it for the visuals, Fate's development and Fate and Nanoha's friendship, or you hated the movie due to bland music and fight sequences, Yuuno basically being written out of the movie, Lindy and Chrono never getting any closure to what happened with the Book of Darkness as well as the fact that the Wolkenritter held onto both the Idiot Ball and the Conflict Ball just to make the plot work.
About the only things universally praised by all fans in the movie was the cleaned up visuals, fight against the Darkness of the Yami no Sho and Reinforce's device being cool. However, the first action sequence is also universally disliked by fans, good luck getting them to agree on the why of that, however.
Narm: In retroperspective, Signum's voice was higher in early episodes, which can feel rather odd. It became deeper after a few episodes, even though not as deep as she sounds today, but the odd feeling is not as strong than in the early episodes.
We never find out how Hayate reacted to hearing about Graham having planned to use her to seal away the Book of Darkness, or if she ever found out at all.
Kyouya and Miyuki are shown to be expert swordsmen in Triangle Heart 3: Sweet Songs Forever. One of the enemies their sister is fighting in A's is a sword master. Despite the obvious opportunity for a duel between Signum and either non-magical Takamachi sibling, the most significant event featuring those two in that season is Miyuki ribbing Kyouya about his love life.
Considering how much Vita prides herself on there being nothing in the world she cannot break, why didn't she ever seek out a rematch with Yuuno Scrya, the boy who — without any Device and lacking her centuries of combat experience — managed to fend her off entirely without even giving her the dignity of his full attention? There's no way that he wouldn't have infuriated her, and no way she would have forgotten or forgiven the offense to her pride. At the very least, she should have become the SECOND antagonistically snarky bane in his life, after Chrono.