Interesting question.
I'm not sure if I can give you a straight answer to it though... but I'll try.
As I see it, most big cities have tons of tourist traps. For instance, when I visited Venice, the prices in the most crowded areas were so high, I though I was gonna faint! It's impossible than a simply key chain cost more than €1 - €2! Yet, I have seen people ask for €10! So, that one tourist trap to definitely avoid!
My advice, if you plan on visiting big cities or, in any case, touristy areas, stay away from the souvenir shops, coffee shops, restaurants, ecc., in crowded areas. In order to not overpay for a meal or anything else for that matter, go off the beaten path and explore the area on your own
Also, I'm pretty sure you're curious about any dangers that tourists may encounter. Well, just like in any other country, there is a risk that someone might steal your wallet when you're not looking. But, you know, this can happen anywhere in Europe... So, don't plan your visit to Italy based on how safe some city is, as long as you're careful, nothing bad can happen.