Must be secure, easy to use, and hard to mess up for less technical users.
- I'm guessing you are dealing with a creative agency, and a lot of media - video/images over here. In that case, I'd suggest trying out hightail. An agency we worked with in the past used hightail to share storyboard and video files with us and we loved the experience.
- It is the simplest free service for sharing files. And I think almost everybody has a gmail account.