As a previous poster already said, its a lot of little things-all together-which make this such a brilliant and addictive game, but some which immediately come to mind are-the multitude of terrain improvements and unit promotions means that players have a VAST number of choices to make in the game pretty much from the get-go (rather than the old Road, Mine, Farm routine); the diplomacy and religion systems really add a whole new exciting dimension to international relations; the ability of smaller nations to actually survive AND prosper is great; the unit-voices, music and voice-over by Leonard Nimoy may just be 'ear candy', but they really add to the immersiveness of the game. Oh and, trust me, if I didn't have to work 8 hours a day-you would probably never have heard me raving about this game either!
Yours,
Aussie_Lurker.