If you're an Outlook veteran recently exiled to Gmail, the service has recently added a feature that should warm your little heart. It's called Preview Pane, and it gives Gmail a three-column view complete with instant previews of your mail messages, just like Outlook does.
To activate Preview Pane in Gmail, click the little gear icon in the upper-right corner of the window, and then click Labs. Scroll down the alphabetical list to the Preview Pane option, and click Enable. Click Save Changes at the top or bottom of the list.
Now go back to the gear menu and click Mail settings. The 7th item down the General tab list (as of this writing, anyway) will be Preview Pane. Next to it, you'll find a dropdown menu that lets you customize the delay before marking a previewed message as read.
Preview Pane mode changes the way Gmail mail management works a little bit. For instance, hitting Enter no longer opens a selected message, because the message is already visible in the preview pane. Otherwise, most of the standard Gmail hotkeys still work as normal.