One cool feature before 1.7 was to specify who was considered a participant in the "site-course", so it appeared in the participants block and so on.
I've been trying to replicate such behaviour in Moodle 1.7 (by giving all the logged users the "Student" role, vía admin/users/permissions/user policies, defaultuserroleid conf variable) and I've detected some problems:
1) With debug on, I see this:
Warning: Invalid argument supplied for foreach() in /Users/stronk7/cvs_moodle/moodle_17_stable/lib/adminlib.php on line 1496
Warning: implode() [function.implode]: Bad arguments. in /Users/stronk7/cvs_moodle/moodle_17_stable/lib/adminlib.php on line 1502
2) Students in some courses continue not viewing the participant list at site level.
3) New users don't see the participant list either.
4) Users are able to enter to any course! So that setting seems really dangerous, while the help text seems to say that it is only one "site" feature.... (although perhaps I'm understanding wrongly the meaning).