Hard luck hero whose sense of responsibility is far greater than even the great power he received in a radiation accident: an accident that on the one hand gave him the power to challenge gods and on the other turned him into a freak feared and shunned by the rest of society.
Orphaned as a child and raised in the outer boroughs of New York by his kind but poor Aunt and Uncle, he grew up to with surprisingly low self-esteem, given to DEEP self-pitying at the smallest set back, but hides his morose-ness behind a cavalier attitude towards the cosmic weirdness that surrounds him and witty banter with even his most deadly foes.
That pretty much sums up both characters there, doesn't it?
But they look different, you might say! Spider-Man is... and while the Thing is made of.... True! But the way they vary in design are precisely the ways they mirror the artists who designed them!
The Thing is stocky, strong, pugnacious and (originally) short... like Jack Kirby!
Spider-Man is lean, intellectual, and (originally) be-speckled... like Steve Ditko!
Which I guess just goes to show: writing yourself as the super-hero of your own story?
Not a new thing.