Worlds on Fire gives you six cool new settings to play in: Tower of the Serpents, White Picket Witches, Fight Fire, Burn Shift, Wild Blue, and Kriegszepplin Valkyrie. I'm a sucker for both pulp (Kriegszepplin Valkyrie) and post-apocalypse (Burn Shift) adventure so I was sold on this book from the get go. Wild Blue's "superheros of the Old West" setting is also lots of fun.
But I'd have to say the setting that stands out the most is Fight Fire. With it's customized skill set, it really highlights the ways you can tweak the Fate system to drill down and focus exactly on what you want your story to be about—in this case putting out fires and rescuing people.
[5 of 5 Stars!]