I first encountered Man-Thing in the Official Handbook to the Marvel Universe. The image of Man-Thing lurching towards me on the page will forever be my image of Man-Thing. I loved him before I ever saw his comic, and was massively disappointed when I read the actual stories.
I think that he's a better character functionally than Swamp Thing. He's also scarier - the tentacles on his face and the large round black eyes would scare the crap out of me coming at me in the night. And the fact that he's drawn to anyone who fears him (which includes pretty much everybody) makes him more terrifying because it creates a cycle - you encounter Man-Thing, you fear him, he chases after you relentlessly until he burns you alive. That, in turn, makes you fear him. But his lack of a mind does make him a poor story engine.
Then again, what's Jason Voorhees or Michael's motivation besides killing anyone they encounter? I think the real problem with Man-Thing is that they try to make him the hero. He's not. His profile and even his motto ("whatever knows fear burns at the touch of the Man-Thing") imply a figure of terror. If I had the series, I would make him the villain, sort of a monthly horror movie. People encounter him, run from him, are destroyed by him. People try to destroy him, but can't. The local town draws Man-Thing to them and he traps them all inside. Maybe even introduce a regular Man-Thing hunter trying to stop him. Oh, to own the rights to that character.