There's a few things I can see straight away with out enlarging it in photoshop.

• Not following light source.
• The inside lines are to harsh and shouldn't be the same shade as the outer line. (The pleats of the skirt and also the suspenders are to dark)
• The middle band of the skirt shouldn't be straight across. It should follow the curve of the body.
• The shading in the waist band is flat. It looks like it was just shaded around the harsh lines of the folds and suspender clasps.
• The bottom is lop sided. So is the start of the waist pleats. - If this was suppose to be an asymmetric design it should be more exaggerated, if not then then it's lop sided.
• The suspender lines are to harsh, making them look jagged. Tiny things need all 6 colors too. Reference the suspenders we have in buckle up. They'll also show you which way the light is coming from.
• The suspenders are also cutting in her neck.
• The ruffles need more than 2 shades for me to tell whats up. They look like they're still being worked on.

Over all. The concept is really cute, and I'd love to see you continue with this piece. :D

If you need visual references, or further explanation to any of the things I mentioned ping me back and I'll see what I can whip up in photo shop.