Linux Mint 12 added the Mint GNOME Shell Extensions to GNOME 3. Here's how to turn them off if you don't like them.
1. Open the Advanced Settings menu (you can access this by clicking on the menu button in the top panel then clicking on the Advanced Settings icon in the dock on the left).
2. Each extension has an On/Off toggle switch.
3. Simply toggle off the extensions you don't want to use.
4. If you change your mind, open the Advanced Settings menu again and toggle the extensions you want back to On.