Barbara Parker, (Arnold Schwartzenegger's ex-girlfriend from long ago) says that he had this uncanny ability to instantly connect with anyone he chose at an intimate level. Many people who come into contact with him report feelings of awe, as though they are in the presence of someone different. This could be partially due to his fame, intimidating physique and wit; but maybe more so with his dyamic, growth oriented nature.
He says something about how as his body grew in size and power so did his spirit, that he started thinking about himself as a "big" person and that other people began treating him differently.
I do think communication skills are are the crux of charisma because people of low or average attractiveness can be considered highly charismatic - eg. Hitler, Churchill, Ghandi. It's the ability to connect combined with strong inner drive.
By comparison, those that are unpopular or uncharismatic might have their problems centred around a type of social misadjustment where they can't get others to feel what they are feeling and perhaps don't have the drive or interest to try?