Jane is a semireformed criminal, not a sociopath. He does have people he cares about, which includes occasionally relative strangers. He has a crude populist streak and likes to use his manipulative abilities to humiliate people he doesn't like, which usually include the overbearing rich. And the merely pompous can get the same treatment. However, the underdogs and outcasts he encounters do not. He has no intention of abiding by the law and common morality vis a vis Red John, and has a fixed determination to murder him in revenge, which is a permanent underlying tension with Teresa Lisbon, the romantic interest. And his semireformation does not extend to taking the laws very seriously, which is why he habitually excludes the team from the loop. He takes justice seriously, by his peculiar standards, another way in which he is decidedly not a sociopath.
The show often suffers from the mentalist using the power of script, but all character-based shows have this tendency. The show is indeed character-based. It's based on the appeal of seeing the mentalist character do his thing. As an antihero, though, suffering a la Tony Hill in Wire in the Blood isn't his character's thing. Indeed, an antihero winning is part of the show's hidden edge.
There is actually a fairly varied diet of hooks, plots, premises, characters and tone to the stand-alone plots. Not so much in what Jane does but in what the guest stars do and how the regulars interact. If one episode involves a recurring character euthanasing himself or another involves a transvestite troop hiding an abused young man, you've got variety. Even if both have Jane knowing who the killer was, keeping from the team and revealing the killer with a flourish. But odd as it always seems, there are those who simply will not care about any but the regular characters in whom they have invested.
As to that, there are actually the usual open-ended serial storylines. And as usual they suck. Red John does indeed have supernatural powers, exercised commonly in killing the CBI supervisors. What a curious hobby, for anyone. Grace van Pelt has turned into an abusive cop; Kimball Cho has fallen for a junky hooker and nobly renounced her; Wayne Grigsby has becomes a father who doesn't love the mother. All these storyline suck. Jane/Lisbon is UST, because. All the character development storylines, like most character development storylines, suck.
Baker's performance is terrific, but he makes it look easy. Plus the trendies who only go for the dark and gritty can't see past the veil of normality.