My thoughts,
I agree with Steve. Many (not all) experienced travelers cut down on travel times in order to get more out of their limited vacation time.
While visiting many seperate cities in the shortest possible time seems like you are getting a lot out of your trip, you lose so much of your time to travel that you don't get to see much.
It's best to map it out and see what your actual schedule would be to get the point.
Am I training? What time does the train depart, how long will it take me to get up, pack, get showered and dressed, check out of the hotel, catch a ride to the station so I can make the train. When does the train arrive? How long will it take me to catch a ride to my lodging, check in, get settled, find something to eat so I can start seeing the area? If you are traveling by air, the same questions apply.
My technique is to keep my trips to a logical regional swath and keep travel times to a minimum. In this case, I would fly into Milan or Venice, see Venice 2 days, Florence 2 days, Rome 5 days and use the remaining days to stops between the three.
I like to keep London with it's own British Isles itinerary and Paris with either an all-France itinerary or coupled with Belgium and Netherlands.