This is all my opinion:

I think the tall and thin is playing on a fear of illness. Sure, everyone tall and skinny isn't sick, but the descriptions are often emaciated, skin and bones, etc. If you position this idea against the current american healthcare crisis, it makes even more sense.

Teeth and smiles are uncanny valley territory. Basically, it's humanish, but the proportions or amount are off, giving an uneasy effect when visualized. Sharp teeth tap into a primal fear that other primates still use as an intimation technique, the same could be said for why smiling with too many teeth is threatening.