Actually, the clenched hand looks fantastic. You don't want the nail-side of the thumb to be
that curved, or at least mine isn't when I try that with my own hand. Just remember that, when together in the palm, the fingers act more like a triangle than a square - wider at the second knuckle than at the fingertips. And you'll see a little of the first segment on the left of your middle/ring/pinky fingers, and on the right of your pointer finger.
As for the one you finished, the thumb is too short. On my hand, my thumb is almost as tall as the second knuckle on my pointer finger when I put them right next to each other, so even when extended, it should still look longer. You may have been going for some foreshortening, but if the thumb is coming towards the viewer, we should see more of the flap of skin between the pointer finger and thumb. Also, no matter how you have the fingers extended, the tendons from every finger (including the thumb) should stick out. (The one on the thumb is really fun to draw, because it has two tendons - one that goes up to the second knuckle and one that goes from the wrist to the base of the thumb - and they form a little well next to the wrist.
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