We here at CMMC are big fans of the Manhunter from Mars. (By "we" I mean the royal "we." There's only just me.)
Now, one thing that seems to be an instant plot hole for J'onn (and arguably any character) is telepathy. J'onn has always suffered from a fuzzily-defined powerset, and this isn't limited to his mental powers. Throughout his history, we've seen the Manhunter from Mars do everything from read thoughts to control minds to erase memories. In 52: World War III he took Black Adam down by barraging him with memories of the tragedy of Mars.
J'onn's telepathic powers showed up very early in his history. In his second appearance ever (Detective Comics #226) he is seen using "Martian molecular hypnosis" to mind control a baseball player:
Dammit, J'onn. Where were you when the Red Sox won the Series?!
In the next issue, Detective #227, he reads a criminal's mind, but then he lays off the telepathy for a while and relies on "Martian superhearing" and his other powers to get information on crooks. Later on his adventures mostly revolve around his not trying to give away his secret identity and the telepathy seems to have fallen by the wayside.
But it's panels like this that irk me:
The market share for luminous paint in that town is off the charts...
The plot revolves around some crooks who fool J'onn into thinking he's no longer affected by fire so that they can later set a trap for him. He could easily have read that man's thoughts to determine a.) he's a crook, and b.) it's not really a match. However, he didn't really use telepathy after that. J'onn seemed to have abandoned his mental powers, the problems with telepathy fell to the wayside, and J'onn resorted to other means to solve crime. In Detective #299, J'onn didn't even bother using his mental powers to determine the identity of a spy; he just went undercover. Was this abandoning of telepathy a conscious change by writer Jack Miller?
Telepathy is a huge plot killer which opens up all sorts of plot holes. Why do any detective work if you can just read minds? (Okay, I guess maybe because telepathy won't stand up in a grand jury because it's hearsay...) In Detective #227, J'onn knows a criminal mastermind is guilty from reading his mind, but can't prove it, so he drives the guy crazy until he confesses. I suppose he could've kept on pulling off stunts like that, but a trick like that can only work just so many times, and it might make our hero look a little cruel after a while. I'm almost wondering if the Silver Age writers wanted to abandon the telepathy entirely. In his early appearances, when his character was being still being developed, J'onn had all sorts of crazy powers, many of which were whittled away. Maybe the telepathy should've gone away, too. It just leads to all sorts of story problems.
If you can mind-control people, then you could stop criminals in their tracks. And not just petty criminals, but supervillains, too. Why doesn't J'onn use this ability all the time? If I had those kinds of powers, I'd lead with a mental assault, not my fists. (Not only that, but since he can go intangible, he never should get hit at all.)
What about simple mind-reading? J'onn tries not to "invade" the minds of others. Since he's an upright guy, that makes sense to me. But in the Silver Age, he didn't mind resorting to a little deviousness to get the job done. I think the whole "ethics" of mind-reading was tacked on later to depower him a bit.
Basically, if you're going to write a chracter with telepathy (or magic), you need some clear-cut rules. Maybe he can only read what someone's currently thinking about. Maybe he has to touch them. Maybe he can't read insane minds. Maybe using telepathy exhausts him and takes him out of battle for a while. Maybe there's the risk of feedback which can shut his mind down. There's got to be limits on power, or you don't have an even battle, and hence, no story. And putting rules (though not necessarily limits) on J'onn's power will get rid of the need for silly ad-hoc plot devices like "telepathic locks" and whatnot.
Either that, or ditch the telepathy entirely.
For Luthor's sake, I hope he chose Master Lock brand