Although my comments could belong under the 2003 film version, I choose rather to make the comparison here because the film, more than anything else, gives reinforcement to the view that there are reasons this original miniseries is 6 hours long.
In the original, there really isn't a wasted minute of it's 6-hour running time. The complexity of this man's situation requires that the story reveals several different conflicts in his life simultaneously, and how they relate and resolve through psychiatry, The Singing Detective writing, his relationships (past and present), and the music that had become so important in his life. For the film version, because most of this can't be explored in such a short amount of time, most of these elements aren't included. As a result, the film is light and detached... and forgettable.
Apart from that. as another reviewer here pointed out, the acting and casting is MUCH MUCH better in this original despite the lack of famous handsome Hollywood faces (the 2003 film features Mel Gibson sporting a bald head piece to look like a 'nerdy' psychiatrist!).
I'm not an easy critic, but this version is in my top five of all time (movies, not TV- it feels more like a movie that TV to me). 10 of 10