Things I think should simply be when it comes to humanizing Toothless:
He has dark skin. That doesn't mean he's black. Skin colors are far more varied than a simple black and white. But he's definitely dark.
He has a triangular face. Pointed chin. Sharp, wide jaw. Rounded cheekbones. Wide nostrils. Flat nose. Like his dragon counterpart.
Exotic looking. He's rare to the area, as a dragon and as a human. Again, humans come in every variation. Get crazy. You don't need to make him inhuman looking to make him look exotic. Which leads me to...
This is human Toothless, not anthromorph-Toothless. So no random dragon ears or scale patches or claws or pointy teeth. If you add those to a "human Toothless", he's no longer a human-Toothless.
Hair is in the air. I like to make it similar to the spines and nubs along the crown of his head, but to each their own.
On that note... hairless. Body-hairless, at least. Very light on the chest hair and arm hair and navel hair.
Age depends on where you personally set it, though I believe it was confirmed he was similar to Hiccup in age.
Attitude matters. Playful and protective.
Accidentally sexy is okay. Intentionally sexy is kinda weird. That's just me. Toothless seems like the kind of guy who doesn't know nor care about his own beauty, whatever it may be.
Muscles, man. (I have no control on how they come out when I try to draw it). He's more broad-shouldered and barrel chested. Not the tallest, but still rather sleek.
Feel free to argue or add. It's just an opinion. :)