I know I was going to talk about Evil Genius first, but I've been playing Age of Empires 3 for a little bit today, and I have to mention how much I like the idea of Home City Levelling. For those of you who haven't played this game, the premise is that you are establishing a colony in the new world, and periodically get shipments from your home city. These shipments vary from troops, to supplies, to wagons that unpack into towers, forts, and factories.
While that is nice in itself, the fun part of this comes when you get enough xp from killing stuff that your home city levels up. Each level allows you to select a new type of shipment to be sent. So, for isntance, if you can choose between shipments of food, wood, gold, or five musketeers, and your home city levels up, you can choose a new shipment and maybe get a couple of outpost wagons, or some artillery as an option.
Now, it gets better. Your home city level saves from skirmish game to skirmish game. So if you play as the Dutch and get a Home City of lvl 35, whenever you play the Dutch, your home city will start at lvl 35. However, if you play the British, you'll have a different Home City level (probably 1 if you're never played as them).
So why do I like this so much? Simple...you get rewarded for playing. I like the fact that the more time you spend on this game, the better shipments you get. Playing the game for a long time helps you play it better, beyond just building up your skills. It's similar to the campaign mode of Battle for Middle Earth 1, where every territory you conquer increases your army's abilities. This is a nice incentive to play and keep playing, kinda like unlockable guns in FPS games. There should be a reward for playing a game beyond the obvious fun you should get from it.