Everything

Just kidding, sort of. I have a sit inside, main reason is they are dryer in the winter. For me the key is to layer, with the outer layer being wind/water resistant. As the day warms up you start taking off layers. I wear neoprene boots sometimes if I'm getting in and out. Wet feet will end your fun quickly when it's below 40 degrees.

It's actually very similar to dressing to set in a deer stand from daylight till lunch.