Well, I got a keyboard protector for mine -- I'm clumsy and don't want to ruin a $2K piece of equipment by spilling water on it -- but have yet to see how it affects typing (keyboard response is really important to me). Also a sleeve and a portable external drive. Matte screen should be scratch-and fingerprint resistance, so no point in a screen protector.
Already have external monitor, keyboard, mouse, speakers, tablet, etc from my desktop, which I'll use when at my desk. I strongly recommend the external keyboard and mouse, since typing / mousing on a laptop atop a desk can be very uncomfortable. Also have a couple laptop bags from the last time I had a laptop. Looking for a small and inexpensive true USB 2.0 hub and will probably try to pick up a decent set of earbuds.
Might make a makeshift stand, but can't really think of anything else one would need.