Rome has lots of history, but everything seems very commercialized and touristy. The Capitoline museum was fantastic but the Colosseum was jam-packed, even early in the morning. It is bigger, noisier, dirtier, and harder to get around than Florence. Still, it has so much history you can't skip it.
Insider Tip:Climb the stairs at the Trevi fountain and continue up past the Medici villa to find a great view of the city from the top of the hill. Plan your visit to the Vatican museum well ahead of time, you don't want to show up only to find out that the museum is closed on your last day in the city.