These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Canon Sue - Tabitha's long lost twin sister, who seems to have been created from a checklist of Mary Sue cliches.
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.
Crowning Moment of Awesome - Saitou goes alone against up against an army of 70,000 bloodthirsty, berserk soldiers, and dies heroically. Big damn hero, even if he was fighting them for his girlfriend and not Tristain.
It's not so much that he went up against an army alone, it's that he disrupted and held them off for four goddamn hours by himself.
And on top of that, he didn't kill a single enemy, only knocked them unconscious. Now thats what I'd like to call awesome!
Die for Our Ship: Siesta treats Saito much better than Louise, but is loathed by fandom for that.
Dismissed Gender / Double Standard / Values Dissonance - Invoked plenty of times to say the least. Although then again Louise is not acting very different from what nobles actually did during Ancient Rome and Middle Ages. At one point killing a commoner (not to mention beating them up if they displeased you) ceased to be a crime at all (that usually didn't apply to townsfolk though, 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 even be successful if Louise was a male...
Macekre: Original light novels are, even though they still aren't free from occasional Fridge Logic concerning their plot, a few levels over the anime adaptation, which had quickly devolved from grey-coloured story with occasional breathers to Tsundere comedy with a lot of Fanservice and something happening in the background. Because of that, some things were slightly changed, and some cut off completely, often resulting in removing heart-warming or awesome moments just to put more female body parts in (a very prominent example being Saito's death, which was the reason why Louise's company had come into contact with Tiffania, and an important part of Louise's character development, being changed into Saito coming back a short while later on his own, and then dragging Louise to Westwoodfor no apparent reason; another fine example is Agnes' behaviour towards Colbert after his faux death, once again an important part of someone's character development (in this case, of two characters, even), being cut completely).
"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."
Louise may also count.
OT3 - Louise/Saito/Siesta is a popular pairing. 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.
For many this already happened after she gave up her nobility to save Tabitha and went uphill from there.
The Scrappy - Louise and Saito both qualify, depending on who you ask.
Tear Jerker - Seeing Tabitha's mother for the first time; emaciated, insane and paranoid. She hurls insults and silverware at her own daughter, making it that much worse when the cause is revealed to have been a glass of cursed wine intended for Tabitha (then Charlotte). Upon realizing what was going on, she drank it herself to protect her daughter.
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 has been outright omitted, and the rest truncated or chopped up. Had the anime remained faithful to the source materials all along... well, too late now, unfortunately.
The manga and novels aren't immune to this criticism either. In fact, the series as a whole often comes under fire for having a terrific premise, and absolutely terrible execution. The fact that almost every character is wholly unlikeable for one reason or another (Love Makes You Crazy and Idiot Ball being the most common reasons) certainly doesn't help matters.
Wangst: The author tries to justify Louise's abuse by showing her poor self esteem...most people don't exactly see that as a good excuse to beat her boyfriend.
You really can't help but feel sorry for 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 too, once you learn of what happened to her parents.
Jerkass Woobie: Louise in the light novels if you can look past her treatment of Saito. She has a HUGE insecurity complex.