1. Story and screenplay - It has one of the best written story ( more credit to manhwa) , but the adaption is just awesome .The problem with these type of shows comes with the screenplay or the budget ...... but the visuals , the action here is soo good .... as same to the Hollywood's level.
2. Acting - The cast , there acting is absolute perfect with no extra corniness . Other than Han Hyo joo the other actors are not too famous , but don't care about that there acting is top notch. Both adult and children roles are performed accurately.
3. Rest things - Like pace or backstories are handled with great use of time parallels in the main leads.
4. Characters to watch for :- THE adult MAIN LEADS (all three of them) with the younger MAIN LEADS (the pair) .