She's Tomoya Okazaki's main love interest, and the one who he ends up with in the anime. She's always been a weak, sickly girl, but possesses an inner strength that barely anyone gets to see. She's unbelievably tolerant and kindhearted (to a fault, at times). Nagisa Okazaki (née Furukawa) is truly one-of-a-kind.
However, with love always comes hate. Nagisa doesn't receive "hate" per se, but there are certainly those who prefer Kyou, Tomoyo, Kotomi, or even Fuko. The reason? Mainly, it's due to her so-called boring personality.
It's easy to see how Nagisa could be considered slightly one-dimensional as a character. Her only "bad" traits include being slightly clumsy, socially aloof, overly submissive, and lacking self-confidence. Even when she receives a spiteful, threatening note, she agrees to meet up with the sender and try to work things out. She even gives in to the request without hesitation after hearing the sender's sob story (spoiler: it all works out in the end). Unfortunately, this angelic behavior of hers may be the reason why she's considered such a plain character. It can make her difficult to relate to on some levels, since the majority of people have misbehaved or acted maliciously one time or another.
On the other hand, these very same traits are why Nagisa is so loved by all her supporters. It's heartwarming to see someone who's nearly flawless, yet still just an average person. Besides her personality, she doesn't really have any exceptional, outstanding talents (like athletics or intellect). She's a reserved, modest girl who excels at homemaking and domestic skills. Basically, Nagisa is the epitome of the perfect Japanese housewife. Her gentle voice and cute, adorable appearance only add to the list of reasons why she's a favorite among many.
So is Nagisa another boring, bland character? Maybe. The very same traits that make some love her are the ones that cause some to dislike or even despise her. In matters like this, it's all up to each person whether they choose to lean towards either side or none at all. (Personally, she's one of my favorite characters in Clannad.) Nevertheless, Nagisa Okazaki will remain as one of the most iconic, dearly beloved anime characters of all time.