Listening and Communication Skills: You will be expected to follow instruction from your supervisor, and your ability to understand those instructions with little or no follow up will be appreciated by your boss. Also, you might need to relate messages or instructions to others based from your supervisor. Good listening and communication skills ensures that the information is relayed correctly.
People/Interpersonal Skills: You will be working directly with at least one person: your boss. But you also might be the first line of communication between your boss and the outside world, potentially speaking with clients, other employees and upper management.
Organizational Skills: Your boss will likely rely on you to keep the working day in order. Scheduling meetings, travel plans, billing, etc. will all require a successful personal assistant to have terrific organizational skills.
Computer Skills: Writing emails, transcribing notes, faxing, copying, and creating spreadsheets or printing documents figure heavily in a personal assistant's day.