One of the most common mistakes in lettering is found within most styles of contrasted type: where people use thick and thin strokes. It's a dead giveaway showing that you don't truly understand typography. If the stroke weight isn't in the right places, your lettering will feel unbalanced. The main culprit (and my biggest pet peeve) is the capital A.
Dayumm girl, did you play Major League Baseball? 'Cause I can't handle those curves. I don't remember exactly when I discovered these tricks about Bézier curves, but let me tell you — they're life-changing. Not only can you work faster but your curves look so much better.