Well, the key property is it doesn't rust :P. 2nd key property is that it is hard to come by. Gold has a big advantage over, say, (picking a random substance that is expensive by weight) printer ink because with printer ink if you don't store it properly it is likely to lose its value. Gold storage can be delegated to any idiot (such as a central bank official, heh heh) and as long as they do not lose it everything will be fine. Doesn't burn, doesn't rust, doesn't dissolve in water.

Anything as difficult to extract as gold is worth about as much (gold isn't especially profitable to mine). Gold just happens to be about as indestructible as anything we know of and have a very convenient value/volume ratio to procure. It is also a pleasing shade of yellow, which counts for something.

If we are trading without me having something you need right now then I need a way to show you that I'm not some layabout asking you to do stuff for free. Gold is a guarantee that if we unwound the chain of trades at some point someone did something real. That sounds really silly, but it turns out that some axiom like that is needed for trading to make sense.