The first export type implemented should be as minimal and human readable as possible (text files for most content, maybe w/ minimal markup). If that's not possible for all data, then standard formats like html or pdf.
After that, go after your biggest competitor. If people know they have a path back to a familiar shore they're less scared to swim in new waters.
After that, go after your biggest competitor. If people know they have a path back to a familiar shore they're less scared to swim in new waters.