I'm a huge fan of Dick Van Dyke not only from his classic sitcom but from "Mary Poppins" and more. So it pains me when I watch "Diagnosis Murder" and see his considerable talent being wasted on cast-off whodunit fluff.
There's a good track record with taking a likable, popular older star and building a mystery series around them (Angela Lansbury, Andy Griffith, Tom Bosley). But sometimes the elements don't come together, and even when you have an old pro like Van Dyke (or, more recently John Larroquette in the ill-conceived "McBride") it falls flat.
What's wrong with "Diagnosis Murder?" Bad writing, worse acting - a supporting cast that has little-to-no chemistry with the leading man, and a star who doesn't seem to have his heart in the work. It's dull and lifeless - Mark Sloan lacks the character of Jessica Fletcher and Matlock. He's practically a void - he presents solutions and solves the cases and is going through the motions.
It's really sad, because Dick Van Dyke can do drama. Just look at his turn as a murderer in "Columbo." But the material here is so far beneath him he can't elevate it. How this lasted eight seasons is beyond me. It should have been put out to pasture long before that.