I have purchased every Civ game in the series with the exception of Colonization and the new Civ5. I have enjoyed every one of them, and I always feel a great sense of accomplishment when I win, because unless you are playing on the easiest settings, or are a Civ Guru, the game can really kick your butt. I will eventually buy Civ 5 as well, and I'm sure by the time I buy it, it will kick my butt as well as the previous ones have. In fact, there will be a Civ 6 someday, because there is an audience out there for it.
I look forward to having Elemental kick my butt at the appropriate difficulty levels someday as well as Civ I, II, II, IV (and MoM) have already done. I look forward to Stardock and it's talented and committed team, from Brad on down, making the game kick my butt.
Re-reading this, there seems to be a central theme, that I want to get my butt kicked by a game. That's true I suppose. I can't really say that I walk away with any kind of feeling of accomplishment or enjoyment if there aren't any "edge of my seat" moments in the game. Points where I can either deliver a master stroke or get shot down in flames. I'm pretty sure the new hires like that feeling in a game as well, so I'm hoping that together they can figure out how to deliver it. MoM fit on 1 or 2 1.44MB floppies, and it was a seriously good game, hard as well if you made bad choices. Funny how a 2GB game can do so many things visually, but not provide the same flow and challenge as that old classic .. yet.