Realism vs. Fun II: Planets

Making alien worlds is very fun.

But how realistic do they need to be for players to have fun? In Star Control, we've opted for cool over realistic since, in all likelyhood, most planets are either just big rocks, big ice balls, or balls of gas.  So having worlds made out of diamonds and rubies and gold and iron and other weird things allows players to constantly discover new things to explore.