In the early episodes, Red John was simply a plot device - something to motivate Patrick Jane to work as a consultant for the CBI. Red John's crimes were horrific but distant things, significant only in the way they shaped Jane's personality.
In the later seasons however, Red John has been transformed into some kind of evil supervillain. He has an army of loyal followers willing to sacrifice themselves without hesitation. He has spies everywhere throughout all levels of law enforcement. He even seems to be such a genius that he can predict Jane's actions and trick him into incredibly elaborate traps.
Originally The Mentalist was a show about Jane - the smartest and most irritating man in the room - solving crimes. Jane's ego and self-centeredness balanced his cleverness and powers of perception. The result was a likable protagonist who was often his own worst enemy. That was precisely what made the program worth watching: you want to see what he's going to do next.
The show's increased focus on Red John has altered that balance. By having Jane trapped and tricked by Red John, the writers have detracted from his cleverness and powers of perception. They've crushed his ego and made something else the center of his attention. They've turned the show into a sort of police version of Moby Dick.
There are still flashes of the old Jane in the newer episodes, but there is less and less of that since they're now packing in double the story load. Each episode must spend time moving the Red John plot forward, and that means the crime plot of the week must be chopped down to it's barest minimum. The murders between mere mortals are now minor distractions from the eternal battle of the titans.
Sadly, the eternal battles of the titans are actually pretty dull to watch, and they go on forever because when one side wins the show is over.
