Pre-ordered it. Best. Civ. Ever.
There are so many refinements and factors that make Civilization 5 an incredible game, so here it goes:
The art style is beautiful; user interface and overall theme is of a sort of 1930s Futurist look, sort of Atlas Shrugged-esque fonts. I forget the actual name of the period, but it's nice. The UI is clean, not clunky so you can see most of the map, and a lot of micromanaging tasks have been streamlined, such as managing happiness for your entire empire, not each city. Also, pollution is gone.
The graphics are top-notch, filled with details that make looking at your empire a feast on the eyes. Little cows eating grass, birds on the shoreline, flying in circles, living forests. And instead of black fog-of-war, you see clouds that mask view until you explore the area. Very nice touch. Each leader now speaks their own language, and has their own set piece, which is interesting.
The music is gorgeous, mostly orchestral and minimalist pieces that change based on what time period you're in, and when you're engaging in war or not. Yeah, that's about it.
Oh, and combat isn't boring anymore. You can't stack units, so most combat takes place on actual battlefields, and with the nice animations, it's fun to watch your little guys die or succeed.
Tons of features I don't have time to writ eabout, but just get it. Multiplayer with you all will be epic.