From the Queen’s Vantage Point:

Many people fell in love with Jan Di, the long suffering, misunderstood, and often denied girlfriend of Jon Pyo of Boys Over Flowers.  However, Koo Hye Sun is no longer Jan Di.  As she so wonderfully displayed in The Musical, this young woman has range.  What a solid performance she gave as Go Eun Bi, the admirer of her nemesis Bae Kang Hee, played by Ok Joo Hyun.  Hye Sun’s performance is highlighted by the way that she can change from simpering school girl to talented starlet in the two pictures.  Jan Di had no real voice in matters.  Eun Bi had mad skills as an actress and singer.  It was almost like watching sisters.

Two of the most obvious characteristics of Koo are her laugh, which does not change or the way she fluffs her hair when frustrated.  No one really cares, however, as these are two of her most endearing qualities.  Go Eun Bi’s outlook is basically a very Pollyanna-ish as she expects everything to work out in the end.  Jan Di’s is mostly a sad sack for whom nothing is working out no matter how hard she tries!  It is hard to believe that this pixie-faced actress is almost 30 years old because she looks so much like a little girl. She is really a baby-faced beauty but she has some very big acting chops.  There is a definite future here that will see her change from nymph to dramatic actress.  Her range is larger than she is tall.  When she acts, she embodies every emotion of her character.
The Queen previously wrote about Boys Over Flowers here, and Secret Garden here.