Fun little starter project you have there. A few tips: Where the circle sections meet the straight section, you should make the connection softer. Secondly: It seems that you have simply outlined a stroked path in Illustrator to create the letterforms. In this case stretch the letterforms vertically up by 10-15% before you outline the stroke, and then scale back down after outlining. That way you adjust for the optical illusion that horizontals look beefier than verticals. And yes, the S and Z are atrocious.

elegant Q.

The Q is great. I also like N, T, V, J, and the overall feel.

You're right, the S and Z need work. Don't make the middle part of the S and Z go completely horizontal -- use a bigger radius on the inner part of the curves, and have them meet diagonally.

I'd bring the middle of the Y, P and R down, and the middle of the B and K up.

The C, E, and F are too narrow. Either widen them, or experiment with more variation in character widths across the entire alphabet.