HAAAAAAAAAAAAAANDS

They are and will be my eternal nemeses. You've done pretty well (especially the non-thumb fingers are very good), but there are a couple of things catching my eye:

The thumb. It seems like the first segment is too long, and the middle segment is too short. If you even out their lengths a bit, it'll look better. I'd also advertise a liiittle bit of curve to the base of the thumb on the outside of the hand, but this is a fairly minor detail. Also, the line on the palm caused by the thumb is probably a bit too pronounced; if you look at your own clenched fist, this line is probably quite subtle.

The other side of the hand. Unless you have freakishly skinny hands (or disproportionately wide knuckles??), it's probably going to be more of a soft outwards curve up to the knuckle, instead of the inwards one you have now. Also, once more on the palm, you can safely get rid of the rightmost line, given that it's not really there on real hands. It also makes the hand look like it has a cliff in the middle. :I

I hope you don't mind, but i've taken the audacious liberty of suggesting where the lines for the thumb and otherhandside COULD be: