As shown by the quote from the previous educator, Sherlock is making an important distinction between two types of crime. Those which seem extraordinary and complicated, he says, are often the easiest to solve, while those which appear very ordinary and commonplace are often the most difficult.
To put this into context, then, Sherlock is suggesting that Jabez Wilson's case will not be very difficult to solve. The extraordinary details of the crime, like the link to red hair and the strange task which Wilson is employed to do, act like red herrings: they are designed to distract attention and make the case appear more sensational and complicated than it really is. This explains why Sherlock asks Watson for only 50 minutes of peace and quiet to fully comprehend the case:
It is quite a three-pipe problem, and I beg that you won't speak to me for fifty minutes.
That Sherlock makes this distinction between crimes reflects his superior skills of detection, and this is one of the story's wider themes. (See the reference link provided for more information.)
Holmes says the following:
"As a rule," said Holmes, "the more bizarre a thing is the less mysterious it proves to be. It is your commonplace, featureless crimes which are really puzzling, just as a commonplace face is the most difficult to identify. But I must be prompt over
He means that if something is common, it is undistinguished. There's nothing to set it apart. If there's nothing to set it apart, how can you solve it or even recognize who did it--or that a crime was done at all? Think of the minor crimes you do every day and you'll get a sense of this: jaywalking, for example, or speeding.
he means that if it is common it is indifferent than plenty of other crimes so ho can you mark it as different or even set it apart from plenty of other crimes that have happened, so how could you solve the mystery or even figure out who did the crime, how could you tell if a crime happened in the first place?! that is what i believe Holmes means when he says it is your commonplace crimes that are truly puzzling because the more bizarre the crime the less mysterious it proves to be, because its not in another criminals usual type of crime scene. for example if we usually have a criminal that usually kidnapped and murdered kids to place them as a possible suspect they would look at his past crimes and relate it to the crime at hand because that is what you would call his area of expertise they wouldn't put him to a crime that involved murder by blender on the hand and it got cut off it would have to be a similar crime