Crud/rust in the tank. Build up inside the carbs (doubt they'll let you take those off so assume they're clogged), oxidation on anything aluminum, rust on anything steel, deterioration on anything rubber. Take a battery yourself so you can hook it up and test electricals. I wouldn't recommend cranking the engine since all the oil has settle to the bottom. Really need to squirt a tad of mystery oil down the spark holes and let it sit for a day or so to loosen up the pistons and rings. O ya, that's another thing. Pull the spark plugs if you can and peer down the holes and see if there is any rust on the cylinder walls.
Get it a TV remote and move it in to the living room for a conversation piece. Often if the gas was not shut off upon storing the bike the floats will stick and then the carbs drain all that gas down to the bottom side of the engine so change the oil as well in addition to what he said about contacts get a piece of light grit sanding paper to clean the contacts on starter button/kill switch signal lights and such. I got a free GS500 because simply the contacts on the starter switch needed to be cleaned. If your bike was in a moisture prone environment its rare but you might want to pull the side of the engine cover with the clutch pack on it and check for rust on the clutch rings. (I didn't check for that one and found out later when I cracked it open after riding it for a bit the clutch was crappy) Check that the throttle/(clutch if its not hydro) cables actuate smooth. You can lube these forget what type but its a real light weight lube. Safe practice if tires are that old swap them out and check wheel bearings while you do it. Last buy a chain cant recommend anybody riding on an unknown chain condition. After 3-600 miles do an oil swap as the new oil will pull any old crap out its kinda like a re-break in on the engine.