When you assign permissions to most elements you are offered granularity for read/write permissions at the role level.
Whether this is LDAP or local, Roles are built by inheriting permissions in a hierarchy.
So the way to default to all be it an app or a dashboard you create a role and join everyone to it... with the exception of the admin. So it's your "default" role say "all_users" which you would base on users or power_user (which is based on users) and the only thing that all_users role does that differs is that it has a default app and that default app has a default dashboard and the permissions for Read on that app and dashboard have been granted to users who posess the all_users role.
CLARIFIED ANSWER:
Assign the Admin role to your admins.
And the admins can then see whatever it is they need to see.
To Assign All users a Default App
As for the default page that is shown for all users...
create a role for all users called say... all_users
That's your default rule. Make your "user" version of the dashboard the default dashboard in the app. Or... just leave it in the menu where it is now. add the Admin version to the menu (or dropdown or whatever).
Now change the permissions on that dashboard so that it's only READable by admins
now login as a user... and you'll see that the standard user just simply can't see the admin version.
There can be only one default page per app... so we can't do a switch depending on the login.
But you can set it up confident that the admins can see everything.
And your users only see what you deliberately assign them to see.
Make sense?
With Splunk... the answer is always "YES!". It just might require more regex than you're prepared for!
