I was prepared for the mud last year, but still surprised by how bad it was. Think a 3" deep, unavoidable mud moat circling each of the tents. You don't want to lug mud-covered shoes for 26 miles! My solution was to wear old shoes (and socks), and change just before the trek to the starting line. I also wore shoe covers over my race shoes until a couple of minutes before the start.

The tents will be crowded, wet, and possibly muddy inside, so I would bring something waterproof like a trash bag to sit on. You may have trouble finding enough space to sit when you get there unless you are in wave 1. Definitely bring warm (disposable) clothes, since you will probably have a while to wait before the start. I wore my rain gear to the starting line, and took it off just before the gun.

ETA: also, you may want to skip the gear check if your hotel is nearby or family/friends can get you dry clothes at the finish. The closest I got to hypothermia last year was while waiting until 45 minutes after finishing to get my clothes from bag check. I would have been much better off heading straight to the T to get back to the hotel.