I echo a number of the comments I read. The feel of the clutch. I remember going to a parking lot with my wife and teaching her to just feather the clutch until the car started moving. Once the car was in motion and the clutch fully disengaged, I told her to press it in with the brake and come to a complete stop. Then start the process over again, and again and again.
I think all too often, people just pop the clutch once they feel the car is in motion.
I learned on my dad's 67 shelby. That clutch was very heavy. My weak legs had issues feathering it. My dad, being impatient, just told me to bring the car up to 3500-4000 rpms and pop it. I learned, albeit improperly, how to start off each time by laying a patch. Probably not the best car to learn how to drive stick in.....
It wasn't until I drove an s-10 pickup with a 4 banger that I realized the art of feathering a clutch.