Well, if you're watching a '96 drama from today's perspective, it is understandable that Long Vacation appears to be overrated to you.
Comparing LV with modern romance dramas, LV's plot may not seem to be as exciting or interesting. However, back then, the plot wasn't the driving force for romance dramas. The audience were more into the characters in the show and the chemistry they had together. People wanted to watch the way they communicate and interact.
This is what makes LV so enjoyable to watch. The undeniable chemistry between Kimura Takuya and Yamaguchi Tomoko. Backed with an excellent cast and very memorable BGM, LV is easily a timeless romance classic for many people back then, especially those who were able to identify themselves with the theme of the show of people being at the crossroads in their lives.
And that brings us to the 'greatness' of Kimura's shows.
He may not be a very technical actor but his charisma, stage presence and particularly, his natural ability to strike a strong chemistry with any of his counterpart, is the charm of his shows. This is why Kimura is able to pull in the ratings, even with a mediocre script. People find it enjoyable and interesting to watch him interact and work with his counterparts.
I used to think that Kimura is overrated as well, until I've watched all his dramas and slowly discovered his strengths and appeal.
-------------------------------------------------------------------------
To stay on topic, I'm currently watching Tiger and Dragon. A really creative and witty drama.