Disclaimer: I cannot summarize something and assess it without having spoilers. You have been informed. Also this is a subjective review, therefore, what I write here is not fact, it's an opinion. Furthermore, this is a very well loved and appreciated movie by most. Hence, my opinion is in the minority.
The movie portrays two groups of people, humans and borrowers. Borrowers are aesthetically human, tiny humans, able to fit in the palm of a normal human hand. The human characters are Sho, Sho's maid (Haru), and Sho's aunt (Sadako). The borrowers are Arrietty, her parents (Pod and Homily), and another borrower boy. The borrowers survive by borrowing things from normal humans' homes, this time around it being from Sho's home. Sho is aware of their existence and one night catches them in the act, but was not able to see them. The main plot of the movie starts then, when Sho tries to aid borrowers in every way possible. Yet instead of accepting Sho's help, they view him with suspicion. The one time Sho and Arrietty come face to face, they engage in a bit of an argument (for a lack of a better word). After this, Sho's maid who also has been aware of the borrowers' existence captures Arrietty's mother, assumedly to make money. Sho saves the borrowers, and the borrowers due to this scare finally decide to leave Sho's home. Arrietty and Sho say goodbye to each other, and never see each other again.
I found the movie to be heartwarming and natural. Yet at the same time it was plain, and dare I say boring. Some would say it is because the movie is a children's film, and I am very well not that, a child. But I do think that is not true because there are several "children's movies" that I have enjoyed and been entertained by. Some of them by Studio Ghibli itself. I think for me there were four main issues that made the movie not as enjoyable.
1. The two main characters Arrietty and Sho did not meet each other face to face very well until the second half of the movie. I feel the purpose for this might have been to make their friendship more meaningful and less superficial. For me this however made their friendship seem less believable because they were not given the time or space to develop that bond.
2. If the movie had come out in the earlier to mid 2000s then I would have appreciated the plot much more even if I watched it now. But this was released in 2010, and I was an anime junkie back then (and a child). Other animated movies from that time were not this simple.
3. There were hints of intriguing subplots and supporting characters, but they were not fleshed out to their full potential. Now I personally enjoy knowing everything about everything and everyone in my stories. But some people only want the focus on the main character(s). To each their own.
4. I think this was perhaps the most disappointing aspect, the movie was adapted from a children's fantasy novel by the name The Borrowers authored by Mary Norton. Having read into the novel's storyline which was very much intriguing, I wish the movie would have take some of the more complex nuances of the book.
Overall, despite the lack of a stimulating plot, the life of the characters makes the movie very charming, the art is undeniably beautiful and reminiscent of innocent times. While not for me, I am certain a lot of people craving for simpler times would enjoy The Secret World of Arrietty very much. And those are my two cents.