Audience-Alienating Premise: A weird example in that it only developed this after the novels, manga and anime were released. Before the first renditions of the series came out, when only the fundamental concept of "Cute Witch accidentally summons an Ordinary High-School Student to her magical world as her familiar", it attracted a lot of positive attention. After it came out, and that premise expanded to describe how the Cute Witch is a violently abusive tsundere and the student a lecherous Jerkass, interest dropped sharply. With the reveal of Saito's Happiness in Slavery which is magically enforced by the familiar spell forcibly inflicting Laser-Guided Amnesia, things got even worse.
Colbert Bump: The series saw a huge upswing in popularity on the Spacebattles Creative Writing board after The Hill of Swords showed up on the Fanfiction Recommendations thread. Prior to that, it had been an almost complete unknown in the community; now SB is effectively the epicenter of the ZnT fanfiction community.
"Tabitha has taken over Siesta as my favorite character, and not just because she is channeling Yuki. She is made of pure win, I don't usually like the stoic silent types, but something about Tabitha just makes my inner moe meter go berserk."
OT3: Louise/Saito/Siesta is a popular pairing. It's likely because Louise and Siesta balance each other out. This was teased a little in the 12th volume where Louise says she would be disappointed if Siesta gave up on winning Saito so easily.
The series as a whole often comes under fire for having a terrific premise, and absolutely terrible execution - mostly due to the Harem antics centered on a boring Ordinary High-School Student everyguy.
The anime works at a conversion ratio of three to four volumes per twelve episodes. Due to this, at least two thirds of the source material have been outright omitted, and the rest truncated or chopped up. Though the manga and novels aren't immune to this criticism, either.
Values Dissonance: While it's meant to be funny, Saito's abuse and overall treatment is horrifying to modern audiences. However, Louise is not acting different from what nobles did during Ancient Rome and Middle Ages. At one point killing a commoner (not to mention beating them up if they displeased you) was not a crime at all (that usually didn't apply to townsfolk, who were under protection of law to the extent that you had to have a valid reason to kill them). One has to wonder if the series would be successful at all if Louise were male.
Wangst: The author tries to justify Louise's abuse by showing her poor self esteem but most people don't see that as a good excuse to beat someone.
What an Idiot: Saito hasn't figured out that commenting about large breasts around Louise (or her lack thereof) is a bad idea.
Saito, because of the abuse heaped upon him by Louise. Hell, even the other familiars feel sorry for Saito, and see his treatment as both a familiar and a human as horrible.
Tabitha qualifies, once you learn of what happened to her parents and what her uncle does to her.
Jerkass Woobie: Louise in the light novels if you can look past her treatment of Saito. She has a HUGE insecurity complex, a horrible school life, and her treatment at home is only better because of one of her elder sisters.
The Mexican Spanish dub does this regarding the nicknames of many of the characters, who received more realistic-sounding nicknames compared with both the original Japanese and English ones, while keeping the meanings intact. Character-wise, Saito, while keeping his characterization from the Japanese version, also sounds like a stereotypical Mexico City teenager, without going overboard with it, and is capable of sounding more angry and freaked out in a somewhat more realistic way than in Japanese. Louise and the nobles, on the other hand, sound more mean and arrogant, possibly because that's how many Mexicans tend to depict people of higher social levels in Mexican media, especially in comedy.