What I like most about Catching Fire is the subtle way the story raises the sense of danger for Katniss. A meeting with President Snow is filled with tension and foreshadowing. The announcement of the Quarter Quell, that past victors will have to compete makes Katniss react so brokenly, we question her sanity. Everything is charged with a sense of finality and doom. It makes Catching Fire seem like a dark shadow of the first book.
The world building in this book is much improved. Katniss narrations are not as "expositional" since we already know what she's talking about. She can revel in little details, which is what I was expecting from The Hunger Games.
The new cast of characters is very fascinating, they are all very deadly indeed. The Quarter Quell arena is something of horrid beauty with exciting new traps. I admit I was entertained by the painful deaths of the victors. Katniss carries herself expertly, using her smarts and reflexes to survive but still being sweetly naive about the motivations and machinations of her fellow contestants. She's a joy to read about.
The ending is a great cliffhanger that made me run to the third book and devour it as quickly as possible.