Zero no Tsukaima is by no means a GREAT anime, but it had the qualities of a GOOD anime. The animation and art style was comedic and fitting of the story, the characters had defined motives, and an ultimate choice was made defining a change in the characters. I found the anime interesting for these points and watched the second season to enjoy them again --except...
The second season, Zero no Tsukaima: Futatsuki no Kishi, threw those qualities away. They changed the tone of the story, art, and animation of the anime from the first season. The tone of the first season was light and comedic which worked well with aspects of the story such as the protagonist's perverseness, the heroine's condescending nature, and the idea of a person being a familiar in a world where creatures are familiars. In the second season, the tone of the animation, art, and story are more thick and serious which work terribly with the anime's whole concept. The protagonist is annoying and no longer funny and the heroine is overly jealous and down right abusive.
The change in character is reset and they act the same way they did before the resolution of the conflict. Additionally, new plot devices were added that diluted the focus of the anime and previous almost Deus Ex Machina devices were humiliatingly weak this time around. Not to mention, when the characters previously had no problems about killing thousands of soldiers, they suddenly have a moral code for no killing without any kind of spark that would've lit such a moral code. What I mean by this is that; Spider-man's motivation for fighting crime is because of his conceited actions that caused Uncle Ben to get killed. --Zero no Tsukaima has nothing like that. Ultimately, the second season is riddled with plot holes that greatly weakens the immersion of an isekai anime.
I have no problem with the studio deciding to pursue a different theme for the second season. However, to completely disregard the first season by breaking the rules it established for itself is completely unacceptable. If the studio wanted to make an anime about an abusive girlfriend who has family problems, they should've made a completely different anime, not disgrace the first season by only loosely following the previous season's plot.