* Shell buffers (input and output history -- including input and output history of the shell, which isn't quite the same thing as the emacs idea of that)
* Command output buffers and other temporary buffers, including *scratch*
* Files that you're looking at that may get changed behind your back that you deliberately haven't reverted yet.
* The state of the lisp world
Obviously, emacs loses all of this if it crashes, but this in the context of choosing to reboot vs. suspend/resume. I'll take my chances on crashes, and I do save my work, but it's still more efficient to keep a session going as long as possible.