Let's get the negative out of the way first -
1) Weak writing and too many episodes. The first episodes were strong, then a big gap in the middle, then the last couple of issues tied everything up.
2) Often felt like a pout-fest. Between Park Shin Hye and Lee Sung Kyung, someone was always pouting.
3) Absolutely didn't buy Uhm Hyo Sub and Jeon Gook Hwan as father and son, there's only a 14-year difference between them.
4) The friend with the restaurant and the guy that quits and hangs out at her house. They could have both been eliminated with no loss of anything important.
5) The ridiculous breakup of the main couple. First, she tells him to change and then she pouts through the next episode. Just seemed like filler to me.
6) They always seemed to be having coffee or drinks throughout the day. I was wondering when they actually worked.

What I liked:
1) The main couple were so freaking cute together! I'm not normally a fan of student-teacher things, but that didn't bother me here. Once the years have passed and the first thing he does are asking if she's married or has a boyfriend, loved it!
2) I LOVED the story line of Yoon Do. Yoon Kyun Sang did a great job. He was true to his feelings throughout the entire drama, but wasn't obnoxious or trying to break up the main couple all the time.
3) I HATED Seo Woo! But that's a sign of good acting. Lee Sung Kyung made this a character you love to hate.
4) I was surprised to see Namkoong Min in episodes 13-15. That was a treat, he's always excellent.
5) Uhm Hyo Sub was also excellent as Seo Woo's father. I hated that character and felt that he got off too easy.
6) A couple of episodes were very touching.
7) Park Shin Hye downplayed the loud out of control crying.
8) Since this is an older drama, it was fun seeing so many actors that I've enjoyed in other projects.

So, it's not rocket science, but there were a lot of times that just made me smile.