-vendors are even more persistent here than in Athens and there are about 5x as many. You have to say "NO." pretty forcefully to get them to leave you alone...and don't risk starting a conversation, unless you are willing to buy something.
-for how crazy the driving appears to be, there seems to be very little angry honking...they must know what they are doing or something! :)
-the week-long (or longer) metro tickets are worth the price! Buy it.
-what appear to be pedestrian walkways, may not be solely for pedestrian use. Chances are that even if you are on a super narrow, cobble-stone road...cars and motorbikes are on it, too. Be vigilant!
-everyone will tell you something different, even if you are asking the exact same question: pick your target wisely. Look for someone who seems like they would have nothing to gain by lying to you (it may sound pessimistic but it is true!)
-cash is king! A lot of places will not take credit cards or bills larger than 20.
-the further you travel away from the touristy parts of Rome, the less English the locals will speak and the more likely it is that they will be annoyed that you are American :P