It sounds like my thesis topic years ago....
Adaptations good or bad depends on whether the works turn out to be what the adapter originally intended it to be and also who are the audience judging the work.
In a non-academic way of arguing, I will cite the Red Cliffs movie as my example. The director claimed that the movie would be historical in terms of plot. Then we have Xiao Qiao all over the place and interrupting what could have been heroic moments for men. Hence I say it was a poorly adapted movie because it was neither historical nor novel like and the focus of the story was just completely off. The movie did not meet my expectation of the claim the director made basically.
Whereas for works that deviate from the original storyline and tries to re-explain character behaviours by inserting sub-plots, different points of view, etc, they can be good or bad adaptations depending on how closely the audience choose to interpret the original plot. The hardcore purists will demand to follow line for line like a Noh play. For them, a good adaptation will be one that is able to show the finest bit of the story and they will watch closely to see if the particular scene appears or not. Some others would prefer a refreshing take of the original as long as new stories are injected within reasonable boundaries of imagination. It is difficult to explain how to define what makes an adaptation faithful/unfaithful to the original, i.e. whether the adaptation has the "essence". Highly debatable in fact.
So for example, character A in the original story is an angry person and would frequently offend people. The adaptation shows Character A to get in trouble with someone important because of his temper. This is somewhat a reasonable assumption in storytelling. But if the adaptation rewrites the part to say character A is so frustrated with the world that he commits suicide, then that's probably too random. There will be loads of explaining to do and the audience may or may not buy it.
And so I conclude... DW is quite a refreshing adaptation of RTK except that the costumes are getting a bit too wayward and the character dialogues are a tad bit cheesy. Entertaining nevertheless.