It certainly is helpful to be a good typist, but believe it or not, this is not the primary requirement.  Medical transcription is not a "keyboard" specialty, but rather is based on a knowledge of medical language.  It is essential to possess good English grammar, punctuation, and spelling skills.  Likewise, the successful transcriptionist must develop excellent listening abilities and what I call "audio acuity".  The ability to discern what the doctor has dictated, often times with significant background noise, is the foundation of being a good medical transcriptionist.

The doctors for whom I work have spent as much as a decade being trained to become physicians.  From that training they have developed awesome knowledge of the human body and, believe me, vocabularies to match.

An important quality that a successful medical transcriptionist must possess is acute attention to detail.  It is essential that every effort be made to get the work right, even if that means spending an hour with your nose stuck in a medical dictionary trying to find that #@%& word which the doctor mumbled at 90 mph with a mouth full of popcorn.  You can't just take a phonetic stab at it hoping for the best.  In medical transcription, guessing "just won't get it"!