I always like the idea that with all supplies such as Iron, Coal etc that you have a certain production from the mines each turn rather than they just sometimes run out at random. What i mean by this is you can create a stockpile of the goods in question as a reserve for what you may want to build at a later date or in another city that does not have the resourse. They also become a lot more tradeble. Rather than simply trading iron with someone you can choose to trade x units of iron.