A bit of Japanese context: Shinkai stated that he wanted this to be a romance based on the traditional Japanese word for love, which means "lonely sadness".
The characters in Makoto Shinkai's works are a peculiar thing to me. I, being a sucker for character development and love stories, was always confused as to why 5 centimetres per second hit me so hard in the feels even though I didn't really feel much for the characters. After rewatching, I found that it was the relationship they had - the things said and left unsaid, the distance between two hearts despite their closeness, all these mushy relationshippy waves - that hit me the most. To me, it's what Shinkai says about humans and their relationships through the characters that matters the most.
Kotonoha no Niwa is not a love story. Well it is, but let me elaborate. It is obvious that the two main characters belong to different worlds; a 15-year-old in high school (Takao) and a 27-year-old living in the adult's world (Yukari). On the inside however, they are closer together, separated only by society's constructs of age and maturity. Takao is mature beyond his years, working part-time and focusing on his dream of shoemaking - but he is stuck in the unfitting adolescent world, isolated by his peers and work. Yukari is the polar opposite: because of her age she is stuck in the mature adult world even though she says "I am no smarter now than I was when I was 15" (other things happen to make this point but I won't spoil). They are both alone in their separate worlds, and it is this loneliness that brings these hearts together, which is a kind of love. But not your kissy, romantic love. The loneliness of course is represented by the rain. As the rainy season comes to an end, as they find they are no longer alone, what happens to their relationship?
The love they feel is less romantic and more of a connection of two hearts in loneliness. To me, it is a rehabilitating love that gives two lonely souls the strength to move forward in a world where people must fit society's shoes of constructs and expectations (I'm sorry, that's pretty cheesy). I think it is this kind of love that is misinterpreted to be a forbidden, romantic love (by viewers and by the characters), which makes the whole movie into an weak melodramatic soppy forbidden love story between two people. Which I guess it's fine to label it as so, because the art and animation are the best I've seen in any anime.
