Lit particles looked really cool. I've been fucking FOREVER since a new graphical feature bragged about in a game impressed me (I think it was the dynamic light and shadow detail added to Source circa the Orange Box), and that impressed me. Come to think of it, it's pretty much always a new lighting/shadowing feature that makes me go "Wow"
Like the first time I saw Max Payne 2's pixel shaders and soft character shadows that appropriately became clearer or fuzzier when I would get closer or farther from the floor as I dived around in slo mo.
Their example was over the top but I can easily see it used for stuff like fire and flares lighting up smoke, or street lamps lighting fog at night.