There's so much to say about what makes this manga great I've got to divide it up into sections.
The Art-
Personally, I love the art. And I really respect manga artists who "update" their style to make it more modern. The author has some really old works, but you can plainly see improvements in the style. It's not blocky or crude, it's detailed and inventive. The clothes, the monsters, the PEOPLE (which I emphasize, because they all are so unique), the backgrounds; there's just so much effort put into this manga.
(Though I do admit...Izark's semi-mullet's kind of...UGH)
The Characters-
Izark is a somewhat typical gary-stu, I do admit. But I love that we get to look into the mind of someone who fears a future laid out for them. He's physically strong and seemingly all around perfect, but he's not. He has his flaws too, and though there's less of them, his insecurities and imperfections make him more human. Noriko is just like most people have already said, she's weak and "just an ordinary girl", but she's strong-willed and tries her best in everything. I often try to picture myself in her situations, what could I do different? I might be slightly physically stronger, but it wouldn't help much. I doubt I could run away much faster, or use a sword any better -- she's from a modern world where fencing is a sport, not something most people have to learn.
The side characters are a lot of fun too, even if you don't become as attached. The story isn't about them, it's about Noriko and Izark. While often side characters help to improve the main characters, in this case they pretty much support them. It would be interesting to see if they were more important, but I'm fine with the way the story is.
The antagonists in general, are just psychotic individuals. There isn't much rhyme or reason to why they do things, and they aren't really explained. I kind of like it that way, that we don't know what makes a person insane.
The Story-
One thing I really enjoy is the language barrier in the beginning. A lot of stories don't attempt it, because it's tough to make it work. More often one of the protagonists can somehow speak the same language (ie Red River, where he just kisses her and she understands the language... yeah, good job getting out of that one). The author did a great job here, and it works perfectly. There isn't too much story spent when she can't speak the language, and later on we still see some of her language mix-ups.
I think, after that, as most people have said, the story isn't all that unique. It's predictable and a little cliche, but what I've learned throughout my years is that it's not necessarily how different a story is that makes it great, but how it was written and portrayed to the readers. And in that fashion, it was done excellently. Compare this shoujo to most other shoujos. Love triangles being a major plot point? Not in this one. Jealousy and mistrust between the couple? Not really. Maybe some people like all that excess drama, but I'm sick and tired of it. It's nice seeing a couple that just loves each other, with no doubts or worries.
The ending, in the back of my mind, always felt a little weak. I wished it were drawn out just a little bit longer to explain things better, but, it was also beautiful in its own way. And it was a little different, so I accept it even if I feel it lacks a little.
In the end, this fictional story was well told and is one of the best manga's I've ever read, and I hope that others can thoroughly enjoy it as well.
Hehe....long comment sorry. TL;DR it's a good read 🙂