If you as an author manage to create a character that explodes the popularity of an archetype, then you deserve kudos regardless of everything else (well, the rest can be evaluated separately). The tsundere archetype became all the hype at the time with Louise being the one to blame. Whether it's her trademark 'b-b-b-baka' or her quirky stuttering, Louise is a character with a strong recognisable personality that became iconic in the anime community. I guess that's also due to the voice actress' great work with Louise which cannot be understated.
Story 8/10
Original at its time (at least for animes), Zero no Tsukaima's story can pretty much be summed up as anime hogwarts on a much lesser scale. It doesn't have the depth nor the intricacies of hogwarts and it doesn't try to but for what it's worth, they did a decent job creating an anime school of magic whilst maintaining its own identity and incorporating traditional anime ideas into its story. I have issues with the later seasons but that's another review.
Characters 8/10
Like I said, even if you hate the characters, you have to give props to the author for creating such an iconic character that became the face of tsundere. Louise is proud, spoiled from a high upbringing and one that viewers may initially dislike, but the author dives into the character to explain why it is she is the way she is as well as giving her development throughout the show to somewhat improve on her negative qualities. Though many will say that she never gets there in the end, Louise at the very least remains true to the essence of her character throughout the show and maintains the attributes that makes her character so iconic.
Artstyle 8/10
Nothing to complain about here. Good for its times.
Music 8/10
The OPs of the show are a guilty pleasure of mine. Kind of like the anime itself.