Though the mysteries in Columbo were somewhat more intricate than those on Monk, the mystery plots in both shows were usually beside the point (when the shows were at their best).
Keeping the plot secondary is a good strategy on these type of shows for several reasons. First off, it’s difficult to come up with even one compelling mystery that keeps the audience guessing; the ground in this genre has been panned over pretty thoroughly since Arthur Conan Doyle, and I’m sceptical anyone can now find a truly unique plotline; it might happen once in a while, but 13 times a season? Second, since you must inevitably recycle basic elements, the temptation is to use new locations, unfamiliar professions, obscure science, etc. to introduce variation in the plot. Exposition for this is often exhausting, and the solution often requires too much explanation; if a murder depends on, say, the details of watchmaking, chances are you’re going to spend a lot of time in a cramped room hearing someone tell you about quartz crystals, spring tension, or whatever, and the solution is going to hinge on some deus ex machina detail about serial number engravings or similar arcana.
The better approach is to treat the plot as a clothesline on which to hang character-driven scenes. Columbo was effective precisely for the fact that you knew from jump “whodunit”, and in most cases how they “done it”; it was the subtle and often complex interplay beteeen the apparently bufoonish Coumbo and the suspect that made for they pay-off. The scenes where Columbo would “stumble” across some fact or item related to the crime, just to get the reaction of the suspect, the “just one more thing, ma’am” needling, the apologetic yet completely objective marshalling of facts in discussing the case with the suspect (I would love it when he’d lay out a few facts, let the suspect make a conclusion, then say something like “you’re probably right sir; oh, but there was this other troubling thing…” which completely unraveled the suspect’s self-serving explanation). The best ones, in my opinion, were when Columbo would get the suspect to admit his/her own guilt, either through some subtle mis-step or pushing hard enough on the suspect’s pride in the “perfect” murder to prove how smart he/she was. Before it became a standard whodunit, and Peter Falk’s character turned into a caricature, Columbo was actually some of the best TV of the 1970’s.
Monk started out with similar promise, but here the key element was Monk overcoming the personal limitations of his handicap rather than the machinations of a suspect, in contrast to Columbo who was playing an elaborate game with the suspect. Getting to the heart of the mystery was a personal struggle, and it was interesting to see how OCD is woven into the fabric of daily life, much less the daily life of a detective. But it seems that Monk only has a finite supply of these personal demons, so the past few seasons have seen more of the side characters (and a brother with a whole new set of prblems; Monks’ Monk, if you will) and a bigger emphasis on solving the mystery. It may seem paradoxic that a mystery show is best when the mystery isn’t the most important thing, but the discussion about the problems with this week’s episode (I didn’t see it, but I don’t think I missed much) seems to prove the point.