I rarely feel the need to describe about a drama. but here I felt necessary to convey my emotions.
This drama just gives you so much feels.. you laugh, you cry, you fight, you hesitate, you smile, you enjoy along with the characters.

Story - It is rather simple story but because it is more of interaction based and character oriented, It never disappoints you. At some points it even surprises you in good way (when Woo jin decided to stay or Seo ri didn't left), OR in bad way ( where Seo ri finds out her uncle had died). But feelings felt 100% real.
You get to see an amazing love story. There is a real relationship, there is care & support for each other and that is what keeps relationship going. Even in real life, you need care & support for each other rather than heartbeats & chemistry.
I have seen many dramas where couple hide many things from each other just not get the other one worrying. But that is where the main problem lies. The relationship falls after hiding things. I just finished watching Something in the Rain two days ago, I didn't really liked it. 30 but 17 feels really different. Woo jin and Seo ri share with each other everything - feelings, thoughts, future plans, even smaller things won't go missing if they are with each other.
Best thing no long term villain or bad character in the drama.

Acting/ Cast - Outstanding performances by everyone in the drama.
Shin hye sun is an amazing actress. I had watched her last in Stranger where her role was very serious which she had done amazingly. I wasn't ready to watch her acting as a 17 yr old. But she felt real as if she was Seo ri. And loved watching her transform from a teenager to a responsible adult.

Yang se jong is fabulous. When I watched him in Temperature of Love, I was rather disappointed in his character. but here I am all for him. Gong Woo jin is my Crescendo.. @_@. His transformation from desolate, miserable and heartbroken person to supporting, open minded and warm-hearted person is the key point for this drama. Woo jin is one of most favorite male leads for me. His undying support & understanding for Seo ri is heart melting. I loved as well as respected his character. I loved when he said to Kim tae hyung - "You never get anxious when you are sure about something." He was that kind of person. He gave time and patience to Seo ri which she needed the most.

Ahn Hyo sub - he is very good as Chan. his care and love for both Mr. Kang and Ajumma is very obvious. you can feel he is a responsible teenager who deeply cares about all the people around him.
Ye ji won - Her character Jennifer is a little closed off and stern. but you can feel she is warm hearted person from her actions. and don't forget her quotes. The quotes REALLY Teach you something good.

Every other character is perfect for the role.
Yoon Chan Young and Park Shi Eun as young Woo jin and Seo ri are just perfect.

Music - Really really amazing.
The songs are very subtle and never felt out of place or unnecessary. "Just stay" is my new repeat.
Other songs - Seventeen, Thirty Waltz & Tears In My Heart
Background music is really perfect.

Rewatch value - Definitely rewatching this. Character development and the whole family grows on you till you miss them.

Some loopholes - As the female lead was unconscious for almost 13 yrs, it felt odd as she woke up as if her brain was alright and in perfect condition. because if her body or brain has no problem then why did she take 13 yrs to wake up. but as this drama tells you - "Don't think, feel". so just don't go deep into loopholes and watch this drama to feel roller coaster ride of emotions.

A Really Really heartwarming drama.. Loved 30 but 17.. <3