Capone's heavy Italian accent and limited schooling.
According to various online sources, Al Capone had a deep voice and he spoke in broken English. When he was raised he was around many native speaking Italians which effected his English. According to the various articles I have been researching, English was a second language for Al Capone. This may have hindered his progression up the ladder of Chicago organized crime, however his brutality was his main asset which allowed him to progress.