Paint - Flux in temps will change the paint and make it unusable.
Firewood - Critters come with firewood. You don't want them in your home!
Canned Goods - Too hot, expand and spoil; too cold could explode. Cans need to be stored between 68 and 85 degrees.
Propane Tanks - Can leak. Fumes can build up and kill you, or a small spark can ignite and boom, no more garage!
Spare Refrigerator - If meant for indoors, your fridge will run extra in the summer and maybe contents will freeze in the winter. However, they DO make fridges that are made to be outside or in a garage.
Wooden Furniture - Humidity and temperature changes can ruin wood, so if you don't want it ruined, store it somewhere else.
Electronics - Temperature changes can cause a buildup of moisture. Moisture ruins electronics.
Gasoline - Fumes are the most flammable part of gasoline. Not good for your garage.