It would be great if access level for attachments could be set. Maybe also for other things like extra fields.

There is a php code snippet that will hide it for non registered users. You can google like: "hide module logged in joomla php". If you know a bit of php you can change it around so instead of getting the user as a member it will get the user as a guest and not echo the attachment.

Access levels for extra fields or groups would be great :)

And what about Tag Groups. It would be nice being able to display certain tag by group in a tag cloud.

That php code snippet USED to be on the old K2 forum on joomlaworks.gr. It appears all those forum posts were deleted. Now I can't figure out how to do it again =(