A bunch- already mentioned- Al trigger guard, heavy ( really heavy) barrel- for durability with muzzle attachments and dent resistance ( sporting shotguns are very thin walled and lightweight), Modified safety and beefed up action bars and heavy walled Al receiver etc... NOT a 500.
Nothing there I would consider paying even an extra 100 bucks for except the larger magazine. I’ve had my 500 for 30 years, put it through a lot use and abuse and there not mark on the trigger guard or anywhere else. It shoots just as hard and accurate as a 590.