When travelling trenitalia, what's the difference between First Class and second? Is it worth the extra money?

There are few seats, so you have a bit more room to move around. You also have a little bit more room for your luggage. All 1st class seats require a reservation, which is at an add'l cost. It's up to you to decide if it's worth it or not. We choose 1st class for the longer trips, say over a couple of hours, otherwise for shorter side trips we usually go 2nd class.

that should read, there are "fewer" seats...

lst class on short journeys say less than 4 hours is a total waste. Yes you have more room but not that much more and its simply not worth the extra cost.

It depends on the train.

If you are taking ES trains, a reservation is mandatory and is included in the fare, whether you travel first or second class.

Are the trains air-conditioned?

It's a matter of personal prference as to when you want to spend the extra money for the room. I would definitely spend it for a 3 or 4 hour trip. I believe it is worth the cost but that's my choice.

If you are traveling on ES train, say Rome to Florence or Venice, then 2nd class is spacious, comfortable, air conditioned, and just fine for the average traveler. The ES trains are all relatively new as well.

If you are oversized and require a somewhat wider seat, then 1st class may be worth the premium.

2nd class is 4 across (2+2) on either side of the aisle, 1st class is 2+1.

Both classes have 2 types of seats. 4 (or 2) facing each other, and in a row. Most of the seats are facing each other, so don't be surprised if you need to share foot space with someone else.

Thanks for all the information. I still need to understand if the trains are air-conditioned.

Yes, trains are air conditioned, except perhaps for "milk" trains.

Even regional trains (like Florence to Pisa) have AC.

Some regional trains are not air conditioned; depends on the region and the service.

Regionals we have taken between Milan and Varenna have never been air conditioned except for open windows.

"Regionals we have taken between Milan and Varenna have never been air conditioned except for open windows."

We used to call that 460 air conditioning in the cars of the 70s.

4 windows open, 60mph.