A hypothetical situation: you've implemented a password handling system, and it doesn't impose any limitations at all on what characters can be used. You want to set up some rules that are a reasonable compromise between two things -

1. Allow the user as much freedom as possible.
2. Allow for the possibility that you may change how you handle passwords in the future - you don't want to rule out reasonable implementations because your users' existing passwords would become invalid.

What rules would you impose? Are there other factors that might affect you choice?