J'onn J'onnz, the Manhunter from Mars, is one of the most powerful heroes of the DC Universe, with super-strength, flight, near-invulnerability, telepathy, shape-changing, intangibility, invisibility, etc. He's one lean, mean, green fighting machine.
There's just one little problem. A boy scout can take him down by rubbing two sticks of kindling together.
It's fire, you see. For all of his powers, J'onn-boy can't take a little open flame, which is why he and Firestorm never sit next to each other at those Justice League reunion barbeques. Considering that the torso part of his costume isn't that much more than a pair of suspenders, you'd think that he'd be okay with a little warmth, but noooooo...
The idea of a hero having an Achilles' Heel is nothing new, of course. It gives writers a quick and easy crutch to use to get out of a creative dry spell. Green Lantern (both the 1940s and the 1960s versions) probably have the most recognized vulnerabilities, to wood and anything coloured yellow, respectively. These weaknesses have been phased out of existence in recent years, presumably due to their inherent lameness. These guys are supposed to be all-powerful, but you can take out both of them with a painted piece of driftwood? Yeah, okay.
For a long time, Superman's only weakness was exposure to several different colours of kryptonite. Green kills, red mutates (temporarily), yellow erases his powers, etc. I half-suspect that the different colours of kryptonite were introduced in an effort to bring at least a little variety to the only-weakness-is-kryptonite routine.
This is my view: if too many years of lazy writing and editing have escalated your hero's powers to the point where he can shift a planet's orbit just by arching an eyebrow, than bloody well write something that knocks him down a peg or two on a more permanent level than temporary exposure to a coloured radioactive rock. It worked for Denny O'Neil when he created a Superman-sand analogue that leeched - permanently - a healthy chunk of Supes' powers away.
The problem with all-powerful heroes is that you need to keep coming up with all-powerful villains for them to fight against. Eventually, there's a disconnect between the readership and the types of battles being written, and the whole series just starts to lose steam. The early comic book heroes resonated so well because they - for the most part - didn't have outlandish abilities and grotesquely far-fetched scenarios to prevail in. Even when Batman's rogues gallery started to shift the emphasis from racketeers and gangsters to the likes of Two-Face and the Joker, the heroes and villains were all still just human, notwithstanding their proclivities and odd obsessions.
Let me put it this way: compared with Matt Wagner's run on 'Sandman Mystery Theatre', a lot of super-hero scripts take not just a back seat, but they're sitting in the cargo section at the back of the station wagon. The guy saving the day without the benefit of powers is always going to be more compelling than the all-powerful titan dealing with an arbitrary, non-sensical weakness. In my opinion, at least.