What makes this a good manga is the steady character development, you can see this by the thoughts and actions of the characters and even the art style, those who have seen the manga could tell the art style from chapter 1 to chapter 110 looks different.
Another point that makes this a good manga is the plot itself, it's original and heartwarming, it's morals are about friendship, changes, family etc.
The problem with this manga is that since this is a romance, shoujo etc. It can get a bit cheesy and predictable. The characters are also a bit generic, Natsume, for example, is your typical tsundere and Mikan is you happy, happy character.
But regardless this manga is one of my favorite, it conveys a special message in the end and has many twists.
