Warm air furnaces tend to be something you really like or you really hate. When you turn on the thermostat the furnace kicks on and heats the air in the plenum. Once it reaches a certain temperature a motor attached to a blower kicks on and blows the hot air into your house via a duct system. Cold air returns in each room return the displaced cold air back to the furnace to also be heated. Warm air systems tend to perform badly or have problems in a couple instances. When an owner has a number of pets and doesn't maintain the air filters the ducts can get fairly nasty and make the warm air from the furnace a fairly unpleasant affair. These type of systems also do extremely poorly in badly insulated houses giving the feeling of a nice blast of warm air and then the cold air sets in again. A lot of houses in the southern United States have furnaces because the ducts can also be used for central air conditioning.

Forced hot water is one of the more versatile heating systems out there. Forced hot water works by sending a loop of hot water through radiation to generate heat. A pump pumps the water through the pipes to deliver heat quickly and efficiently. You can heat different rooms or areas to different temperatures by sending a separate loop of heat to each area you want to heat. These areas are called zones and allow you to control the temperature of your house better than almost any other heating system. Besides the yearly checkup forced hot water requires almost zero maintenance.
 

Steam isn't installed anymore because of the enormous amount of labor required to thread hundreds of feet of black iron pipe. Most likely if you have a steam system in your house the age of system might have given you a bad impression of the steam system in general. Most steam systems are either one pipe or two pipe systems.In a one pipe steam system the hot steam and the cold condensate travel in the same pipe going to opposite directions. This brings us to the first problem many steam owners have, knocking pipes. Unfortunately the main reason pipes knock is because years ago the installer didn't pitch the pipes correctly. If the knocking just started there's a decent possibly they're knocking because the return vents, in the basement, are plugged up. The second type of steam system has a separate pipe for condensate and the steam so the knocking problem is a non-issue with a two pipe system. Steam systems require a lot of upkeep including filling the boiler once or twice a week, replacing steam vents, and flushing the low water cut off when the sight glass looks dirty. 

Gravity systems aren't installed anymore for the same steam isn't installed anymore, cost. Gravity systems use very large pipe sizes so the heated boiler water can circulate around the system via gravity. Over the years a lot of old gravity systems have had pumps installed on them, to speed up the circulation. I've ripped out a lot of the larger pipes and reduced them down to make a gravity system more like a forced hot water one. This usually saves customers a lot of money because they're heating a much smaller amount of water.