I like the Default Group global setting, but really each user should be able to select their own default.
This requires a database change adding to TABLE_USERS. Surprisingly, this works and the option crudely appears in the Personal Settings page.
I have implemented this via a plugin (flies attached). The user can set a default group in Personal Settings.
I have not worked out how to change loib\frontEnd\PageOptions.class.php to make it pretty. (not sure it can be done via a plugin). It really should present with a better label (like in the Admin panel. Also should be a populated drop down list.
Hoping this can be rolled into SVN as part of trunk because it is not perfectly suited for plugin.