Gnome Shell contains various features which are not easily discoverable. I think it would benefit some new users if there was a quick introduction that ran on first login and demonstrated a few of the shortcuts that are available.

I'm thinking:
  * The Windows or Alt+F1 shortcut to the activities view
  * Alt-` to switch app windows
  * The notification tray
  * How to power off
  * The top bar icons (maybe this is too obvious)
  * How to open Help, or rerun the tutorial

Possible others (maybe these are too many / too advanced)
  * Alt+F2
  * Workspaces and workspace switching
  * Ctrl+click to start a new app window
  * IM status menu
  * A "know your rights" link to an explanation of free software, like Firefox are doing
  * A link to extensions.gnome.org

Other concerns:
  * It should be skippable, and available to re-run later
  * It should be quick (maybe 5 or 6 screens?) and it should be clear to the user that it's quick (have a progress indicator, perhaps)
  * I picture this as being interactive, with big pointy arrows annotating the UI

Some users will skip any sort of splash screen, so we can't expect this to be the only way for users to learn to how use Gnome Shell. I see a bunch of use cases though:

* Basic users, who are careful to read documentation before using a new system, but don't know how to activate Help in Gnome 3.

* Certain intermediate users who don't like reading long help files, but will go through a quick tutorial. I'm essentially thinking of my 12-year-old self as an example.

* Advanced users who have upgraded to Gnome 3 but haven't participated in the design of the Shell. I have known several people who actually participate in Gnome itself but have not been aware of Alt-` or how to power off.

Since most (all?) users get Gnome through a distribution, first login is probably the best way to reach them and show them how to make the most of the shell.