Many years ago there were seemingly only dozens of websites covering the butterfly stroke and not the many hundreds there are today. It was back then that I became interested in the butterfly stroke and teaching myself was particularly hard because of misinformation combined with my own limitations. But eventually I learned and wrote things down to work on. I never lost my interest in either watching butterfly swimmers nor in swimming it myself. Alas, I'm not a swimming coach or a great swimmer, but I pay undue attention to everything butterfly.
I believe every aspect of the stroke is critical to one's success. If one has a good coach, the key would have to be listening. I am forever be amazed and humbled that 11 and 12 year old boys and girls are already competing in 200 meter butterfly! It's key is to be physically in shape for fly so everything can be executed correctly. Thorough knowledge of proper technique and awareness of what one is doing right and wrong is essential. Figuring out what is working good and what isn't and doing something about it is up there. Early on I took in some incorrect information, and I misinterpreted or forgot many things which greatly delayed my progress. I was never fond of drills. I learned the hard way which thankfully doesn't short you on fitness. But if only I knew at the beginning what I know now!