What remark does Holmes make about commonplace crime? And what does it mean?

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
this matter."

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.