Angst? What Angst?: Reconstructed but not at first glance. It appears as though Saito doesn't have any angst about basically being kidnapped and enslaved but that is because part of the familiar contract is brainwashing that suppresses it. When the contract is temporarily gone, the angst comes in.
He also doesn't seem to have any angst over any friends or family he may have had back in his world and whether or not he'll ever see them again.
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. Thankfully, this was fixed in Volume 11.
Another weird example is found not in the series itself, but in the European Spanish dub, which was advertised in true Dancing Bear fashion for being performed by amateur voice actors in the main cast roles. The final result (not only limited to the actors, but also to the translation and adaptation) generated a deep Broken Base.
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.
Designated Hero: Louise herself. She abuses Saito and treats him like dirt for the slightest to stupidest reasons non-stop to the point where she may as well punish him for breathing, and is rude, obnoxious and unpleasant to other females usually out of jealousy and arrogance, and she is never punished for her actions. Yet we're still expected to root for her for some reason.
Saito arguably may also count after his attempted rape of Louise.
"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 focus being on the Harem antics centered on a boring Ordinary High-School Student everyguy. All the things people like about it are just the window dressing.
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.
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).
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. This interpretation goes away when the full details of why she is the way she is are explored in full.
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. Unfortunately, he can also be callous and cruel in his own right in response to his treatment, which can kill any sympathy for him.
Tabitha qualifies, once you learn of what happened to her parents and what her uncle does to her.
Unintentionally Unsympathetic: Louise, unsurprisingly. We're supposed to sympathize with her because of her enormous insecurity complex, her miserable school life, an abusive older sister, and emotionally distant and incredibly strict parents who are more interested in their daughter as an asset than as a person. Not to mention that being a noblewoman of minimal magical talent makes her a complete laughingstock due by the norms of Halkeginia society. So while Louise has plenty reasons for the audience to side with her, she still drives off any supporters due to her infamously brutal and non-stop abuse of Saito; which is not only horrible on a basic human level, but also makes her a blatant hypocrite given her home life.
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.
WTH, Casting Agency?: The European Spanish dub was made with amateur voice actors, something that was highly controversial. The fact that said voice actors also did the critically panned 2013 redubbing of Elfen Lied didn't help.