One of the coolest things about Paris has to be their rooftops!
I used to be lucky enough to live in Paris for 5 years and be able to admire the rooftops. There's something extraordinary about them, they're almost as pretty as the buildings :)
It also doesn't hurt that when Napoleon III asked Baron Haussmann to renovate Paris, he did so in a way that all the streets and buildings in Paris are aligned. Haussmann's urban planning included wide avenues and open spaces. I think that's the key to Paris' beauty.
By the way, if you're wondering about the images above, they're in Fabrice Moireau's sketchbook called "Rooftops of Paris".
[photos seen via Paris Breakfast]