First off, the anime series is really, really different. Everyone's personality is the same, but the plot and ending are completely changed after the first few chapters. The anime makes it into a rivalry-driven magical-girl show, but the manga is really just a schoolyard romance with some magic thrown in.
This series is just... cute. The series focuses on the love woes of the main characters and the troubles brought about by Nina's seeming incompetence. The main character (Ayu) is really dull, as is her boyfriend, but watching Nina's interactions with her 3 love interests is really the most fun part of the story.
The author said at the end that she wasn't really interested in fantasy stories after all, and you can tell, because there is nothing special or complicated or very systematic about magic. Furthermore, the characters' personalities are simple, no one has a hidden past or deep secrets. The closest thing to this is Sayaka:
She's a half-witch who suffers because her human friends didn't accept her and she couldn't live in the witch world. Nina's presence makes her unhappy because she's jealous and lonely. She gets in the way of everyone's romances, but is not a real obstacle. But eventually she softens up and turns into a good girl.
This really is a series you should read if you are looking for something light and fluffy.
As for the ending (minor spoilers),
I wasn't shocked that Nina turned out to be really powerful (you can guess it especially when she blasts a hole in the ceiling), but you don't really get to see her doing any magic after she graduates, which is kind of disappointing.
The author chooses to ignore the angst of a long separation and of having to choose between one kingdom or the other. The series had lots of potential for romantic angst but decided to keep it on the light side.
everything works out so well that there's not much tension...