Kanha:
Of course the actors are very important too, it´s not easy spending hours watching an actor that shows no appeal to you. It would be too much of a waste of time watching a drama with a boring plot and actors that doesn´t even capture your interest.
But then, what kind of appeal are we talking about? There are plenty of shows I've dropped or had a hard time watching because I just couldn't stand the main leads, an example would be Kill me, Heal me. I loved and finished that drama and found that Ji Sung was an excellent actor in all of his "personalities" though I had a hard time with Hwang Jung Eum, I found her character/acting to be a bit too much, and that in most dramas I've seen her play in, though to be honest it could be more because of the character (she tend to have especially weird roles I guess). I think the appeal of an actor/actress is, again, mainly his/her acting and that can be influenced by what role (s)he has in a drama/movie.
Recently I finished Just Between Lovers. I didn't like Kang Han Na's (Jung Yoo Jin) acting, I found her character and acting to be mostly bland and yet I do believe she's gorgeous. Then I recently started watching the drama Mother (Korean version) and she had more of a guest/support role but I recognized her and her acting in there was actually good, it didn't seem bland as it was in Just Between Lovers, I thought then she could have a main role and I could like it.
I think the character an actor/actress play as is a very important factor when it comes to appeal and it has to do with the story and not really the actor itself.