The user browser's auto-complete feature is great, but can be even better. I propose improving it (along with a setting) to enable searching users based on their properties. A simple use case could be that we have an additional username (or say why not a phone number) attached to the user as a property. When selecting a user using the auto-complete functionality one would want to be able to use these properties when searching a user.

Say John Smith (jsmith) has a phone number defined using the property "phoneNumber" with a value "12345". When searching for him using the auto-complete feature, one could use any of the following (the last one being the new feature):
John
Smith
jsm
123

This should be relatively easy to implement, as this only requires adding a bit of code to the DefaultUserPickerSearchService's userMatches function. The user object is already fetched so we already have the properties in the object.

Adding a setting to enable/disable searching these properties would require a bit more coding but would still be fairly trivial to implement.