In short nearly everything comes off our boat except lead acid batteries, fire extinguishers and hard mounted electronics..
We remove all cushions, sheets, sails, all soft goods such as dock lines, jackets, anchor rode, anything that can freeze, cleaners, all food, bilge water, tools fenders, all contents of the head medicine cabinet etc. etc....
We have good storage in the barn and it is very dry, better ventilated and leaves the cushions smelling fresh. Removing all this stuff, especially the soft goods, also keeps musty odors down and also helps prevent mold growth. Items that may have salt on them eg: sails, canvas, dock lines etc. etc., if left on board, will simply attract moisture and promote mold and musty odors. You'd be amazed at how much mold grows in the anchor locker when we leave the rope rode in there. I simply drop the rode into an large rubber maid..
Cleaning all this crap off the boat also creates a lot nicer work environment for winter projects.

In the spring it also forces us to re-load only the items we actually use or need on-board. It is amazing how much accumulates over a season that really does not need to be there.