I would say that One Day possesses strong plot lines, each is carefully thought and meticulously written to give the best reading experience to the readers and that was exactly the thing I had gotten! I think it isn't too much to say that Nicholls is such a superb storyteller in One Day and what he has done with this book is simply amazing. He brings the story to life! It certainly doesn't feel like reading some made-up, fabricated story. In this case, it's the other way around. It's impossible not to notice that Nicholls has his way of making as if Emma and Dexter are telling the story rather than the author himself.
I liked the nature of Emma and Dexter's friendship as well. Because they tried to keep in touch with each other and then they had arguments over something petty or serious that led them to not talking to each other but made up eventually in the end, these events bring the authentic value to this story. And I liked the fact that Nicholls doesn't abandon the value of friendship that resonates through out the novel even though this is supposedly to be a romance novel.
If you like The Fault in Our Stars by John Green or Cecelia Ahern's Where Rainbow Ends, I think you would like One Day. Overall, great reading! :D