Well, it isn't a matter of weight. But unlike other parts of the world, in U.S.A. and Canada the heavywight cars are associate with old cars and lightweight with new cars. Something like in most countries of Continental Europe will be the cars before the 25 meters (75 feet long) ones (or close to that) becamed standard in the '50's and '60's.
Note: I will ask you not to compare heavyweight with dome cars. Dome cars dind't existed in the pre-lightweight cars era, so a comparation will be wrong.
From a person who is borned at the end of 1982 and never set foot in U.S.A. or Canada, so never had any real life experience with these cars, I think opinions can begin with:
I. Heavywight:
1) Some of them had the look - it that a train or a mansion?;
2) Most of them had windows than could be open or air intakers on the roof;
Downs:
1) Not all had air condition;
2) Some of them where made of wood (but I think wooden cars from passanger service gone by the '30's);
3) Coach seats where not necesarly confortable.

II. Lightweight:
1) Air condition;
2) Better isulation;
3) More confortable coach seats;
4) Better ilumination during the night;
5) Sometimes larger windows = better view.
Downs:
1) You couldn't open the windows - on coach cars I can understeand it, but at cars with individual rooms...;
2) The inside look wasn't anything but avarage. Well, the old look of the inside was for the one who do apreicate some old styles.

This are merly guidelines for starting opinions. The ones who rode them could say more.
I appologize if I misspelled words.

Heavyweight's ride like a Hudson Hornet...Solid, limited road noise, comfortable seating and lighting.
Just my age impacting my preference ....lightweights are nice but do not have the class of heavyweights.

First, "heavyweight cars" generally refers to steel cars built between 1907 and 1948, generally of riveted construction. PRR referred to them as "standardweight."

Wood cars were significantly lighter.

"Lightweight cars" refers to cars built 1933-present of modern materials, usually (but not always) of welded construction.

RDG and CNJ heavyweight cars did not ride all that well but PRR's P70FBR standardweight cars were rebuilt with new trucks in the late 1940's and early 1950's and were among the best riding cars anywhere.

RDG combine 408 received air conditioning in the 1930's but when it became the recording car for the Iron Horse Rambles and was the first car behind the steam engine, the A/C was deactivated and the latches restored so the windows could open.

If ANY window were opened on an air-conditioned car it would quickly bring the car to the outside temperature and defeat the air conditioning in addition to letting flying insects into the car.

Phil Mulligan

Still I wonder, if it's not a coach car (meaning it resembles interuor a bus, not having compartments *) how can you bring the whole car to a certain temperature.

* meaning any kind of separate room for passanger acomodation, not the specific "Pullman" compartment.

In a lightweight car. each room is fed cooled air from a common duct of cooled air. The occupant can vary the air flow by opening or closing a damper for the room. There was also a floor heat thermostat.

In short, the passenger determines the temperature in the room.

Phil Mulligan

Keep in mind, air conditioning is more than cooling, it also dehumidifies the air, which is especially important east of the Mississippi, which tends to be unbearably humid during much of the summer. The de-humidification takes place when the air flows over the cooling source, lowering its temperature below its dew point, so water condenses out and is drained off. This works best if a closed volume of air is cycled repeatedly, each time getting drier. Opening a window somewhere in the car would defeat this purpose, adding moist air back into the mix. Thus the windows are sealed.

EJ Berry, but if you do close the ventilation, no "poluted" air will get back in the pipe.
I'm curios if after the introduction of the air condition (not just ice cooling) on the heavyweight cars, the windows where blocked.
I don't think I could stay (even in summer) for more the 24 hours without any fresh air. Air condition in hot summer days and in humid areas is good, gets confort, but you still do need some real air.

Pullman's ice air conditioning WAS real air conditioning. The air wasn't just passed over the ice; rather the ice melt water was pumped up to a heat exchanger coil in the ductwork, where it functioned exactly the same as the Freon coil in a mechanical system. Because the inside air couldn't contact the ice it didn't pick up additional moisture, so the ice system had the same de-humidifying effect as the other mechanical systems.

The damper is on the duct from the air conditioner to the room. They usually don't close completely. The carbon dioxide exhaled by the passenger(s) passes under the door, or through the vent in the door, into the aisle and eventually back to the air conditioner, to be mixed with outside air there.

As we had mentioned before, the windows are sealed on both heavyweight and lightweight cars. To do otherwise would defeat the air conditioning.

Amtrak schedules smoker breaks for its long distance trains. If you move away from the smokers, you can get your fresh air. There is no smoking on the train at any time.

Phil Mulligan

Just wanted to make a point: Wood cars are NOT heavyweight cars. The railroads called wood cars, wood cars and they were made of wood. The heavyweights that you are familiar with were always steel. The railoads and Pullman called the steel cars "Standard cars" up until the point that the "lightweight" cars were invented.
At that point you had three categories" wood, heavyweight and lightweight.
T7

And later wood cars have steel underframes so they better deal with slack action of heavyweight steel cars behind them.

Phil Mulligan

They could seal those thing.
Anyway, since vestibule wasn't air conditioned, unwanted air could camed from there too.
Not having the option to open the window... tempted to get an automobile.
Oh, one day I waited in a station durring summer in the train 20 minutes without air conditioning. No windows opening (there where small sections, but you have to have a sqaure key). Very hot. Well, the minutes should have had been 10, but we swithch from Diesel-Electric to electric. The electrice locomotive that they 1st bought had problems, so we had to wait for the 2nd....