Three strong points of <500 Days of Summer> (This is not wrong, but usually if the beginning of the sentence is a number, you should spell out the word)

I just want to tell you about a movie titled ‘500 days of summer’. (title can be a verb in this situation)

Before I watched this one, I thought the movie was about someone's memory of summer days but it is about the memory of a boy who had a girlfriend named ‘Summer’.

This is the story of Tome and Summer.

Whatever you are imagining, this movie will betray your expectations because it is not a typical romantic movie.

I’m don't want this to be a spoiler, so I can’t tell you the crucial parts.

Instead of that, I will point out 3 strong points.

First, the movie has good main characters.

That’s why it’s easy to get immersed in this movie.

The acting is great and their words are connotative and meaningful. (the characters don't act well, the actors do, so I changed the wording)

Second, you can listen to a high-taste original sound track. (English-speaking countries don't use the term OST really, so it's not an official term. Lowercase is okay.)

The director of this movie, Marc Webb, has made a lots of music videos.

I really love every single song in this movie, especially ‘Here Comes Your Man’. (the whole title should be capitalized)

It was not too attractive and not too ugly.

It seemed just normal, but wearable and comfortable, so it makes it seem more realistic.

Lastly, I worry that maybe you won't understand Summer’s behavior or words easily. ("lastly" makes it sound like this is one of the 3 good points in your list)

But this movie shows a realistic relationship well with misunderstandings and expectations, because you want to believe in a the good way while you are falling in love.