Using the Shift key does not provide text selection in the e-mail viewer. In some other content viewers, you can click somewhere or select something, and you can then hold Shift + (optionally Ctrl +) left, right, home, etc. to select text. I find this feature useful.
Some other popular content viewers that support this are Firefox, Chrome and Microsoft Word.
I use Shift and Ctrl-Shift along with the arrow, page up/down, home/end keys extensively in other applications to select a range of items to which I want to apply an action. I really really really miss this capability in the message viewer. (I know that this feature works in the message writer as well as in message lists.
I think the underlying issue here is that the message viewer does not have a cursor. In other words, you can’t navigate through the text using the keyboard. The keyboard instead is only available for scrolling the pane.
I also wish I could double-click on a word in the message viewer to select it quickly. Oftentimes, all I want to do is select and then copy a word or group of words out of the message, and a double-click is how I make (or start) the selection.