Diagram of the parts of a lath and plaster wall. The base coat may also have horse hair mixed into it for stability.

It's easy to forget that drywall hasn't always been around in the home building process.  Drywall techniques today are exponentially cheaper and faster for finishing a wall as compared to the old plaster methods.  When you have a house that was built anywhere around the 1930's and earlier it is likely they used plaster for construction.

Since there is so much material on the walls it makes them extremely heavy.  Even a small garbage bag of plaster can be too much for one person to carry.  Plaster on a wall like this weighs about 10 pounds per square foot.  That means for a wall that is 9 feet tall and 15 feet wide you will remove: 9 * 15 = 135 * 10 = 1350 pounds.  And that's just one side!  Double that for removing plaster from both sides and you end up with literally more than a ton of plaster.  All that lath and plaster material will need to be disposed of so expect a few trips to the dumpster or landfill.  The lath is pretty light and it's extremely dry, so if you have a campfire it can make for good burning material.

Pros:

• Your house is built like a tank.  Plaster walls will keep the house rigid and if all the interior walls are plaster it will help the general structure keep its shape in high winds (tornadoes).
• Plaster walls are better at soundproofing.  If you want to keep sound from traveling, there's nothing like two thick layers of plaster, two walls of slats holding them together, with insulation between.
• Some people believe there is historic value in upholding the original plaster walls.  I would also include with this the idea that the old walls can be a charming characteristic of an old house.
• Plaster walls are pretty good at preventing moisture from entering the wall (such as steam in a bathroom).  However, moisture on the inside of the wall (from plumbing or roof leaks) can disintigrate the "keys" between the wood laths, causing the plaster walls to lose their hold against the wall.

Cons:

• It's much more difficult to hang pictures, shelves ...etc. on a plaster wall versus a drywall.
• Any major problems in the plaster wall is going to be very expensive to repair.
• It will be more difficult to install new electrical fixtures in the wall.
• Since plaster walls can weigh a ton (literally) it puts a lot of stress on the floor joists and supporting structure.  Insufficient support or notched out floor joists can create a weak point which will eventually start sagging.
• A few hits in the right place and the whole wall of plaster is capable of falling off.  You'll see this when you start using a hammer during demolition.
• Old wiring behind the wall is usually knob & tube which uses cloth coated wires.  Although it shouldn't be a problem to leave it, it's best if you can replace it with safer, more modern wiring. 
• Old insulation behind the walls is probably not very efficient.  Over the years it can compact down against the lower end of the wall which creates an empty pocket at the top.

Dust mask or respirator:  You must use a good dust mask (3M makes good ones) so that you do not breath in any of the old plaster dust which may have lead or asbestos.

Crowbar : For pulling the wood laths out of the studs or for

Hammer : Used for breaking up the plaster and cracking the keys so the plaster can be easily removed from the wall.

Sledge Hammer : useful for quickly breaking down the wood lath behind the plaster.  It will be faster than a regular hammer.

Plastic sheeting or tarps: the plastic can be laid down on the floor and around doorways to isolate your work area.

Spray bottle: It is useful for misting the plaster wall to reduce the dust in the air.

Damp mop or Swiffer wet jet : this can be used to clean up dust on the floor after the job is done.

Reciprocating saw (sawzall) : This can be useful for removing large areas of plaster wall in square chunks which are more manageable for disposal.

Before you begin removing any plaster walls there is some useful preparation work you can do first. 

Isolate your work:  There will be a lot of dust when removing plaster, so you want to try to compartmentalize your work.  Purchase a roll of plastic sheet from the hardware store or you can use a tarp.  This should be hung in doorways and laid out on the floor to keep the dust contained.  The plastic can be laid out below the wall to catch debris as well.  Another technique is to hang a damp sheet in the doorways to catch any plaster dust floating around.

Mist the wall with a spray bottle: This is optional, but can be useful to prevent some of the plaster dust from getting around the air.  Definitely use plastic on the floor if you use this method as it will create a bit of a mess.  You should also spray the wall a couple times during the removal of the plaster.

Ventilation : It can be useful if you can open a window and put a fan in the opening pointing outside.  The airflow will push some of the plaster dust outside.

Start by lightly hammering all over the wall.  This will dislodge the keys from the wood laths so that you can quickly remove the plaster coating.  As the keys start breaking you will hear them drop to the floor between the walls (unless there is insulation in the walls).   Don't pound too hard on the walls because there could be lots of things behind the wall that you do not want to break.

After pounding on the wall the plaster will start to buckle and belly out.  At this point, if there is not a hold in the plaster yet you can pound a few times in one area to create a hole.  Use a crobar to start pulling out the plaster in large chunks.  Large pieces will hit the floor so if you're afraid of putting dents on the floor (or breaking floor tiles) it can be useful to put down a sheet of plywood to catch the debris.

You may find horse hair in the base layer of plaster holding it together.  In our house we found something more like pieces of thread throughout the plaster.  Maybe the threads are better suited to the job ... or maybe they got tired of having hairless horses all over the place.  I can't imagine being taken seriously riding into town on a shaved horse :)

Sometimes you will find wire mesh behind the wall supporting the plaster.  For removal this can be very time consuming.  Luckily we only found one portion of the wall with wire mesh (the portion in front of the drain pipe).  Remove the plaster as normal with a hammer, then you'll need to use a hammer to pry off each of the nails holding onto the mesh.  If the rest of the mesh travels up the wall into the ceiling you will need to cut it with a wire snipper.

At this point you will have a wall of wood laths visible.  You should be able to see through them to see if there is any plumbing or electrical behind.  If there is plumbing take special care not to pound to hard with a hammer.  Areas with plumbing can be removed with a crowbar or hitting the lath in an area where the plumbing does not exist.  On areas that have nothing behind you can feel free to hit with a sledge hammer for quick removal.  Just be careful not to hit so hard that the sledge hits the other side of the wall as this will dislodge plaster on that side also. 

Once the entire wall is cleared it is a good idea to do some cleanup.  Use contractor grade garbage bags (3 mil thickness) to hold the plaster as it will be very heavy and plaster shards can poke through the bag creating tears.  As you start bagging the plaster try lifting the bag to make sure it doesn't get too heavy.  This can get heavy very quickly so you won't be able to fill the bag even to halfway before it's too heavy to move.  Aim for about 50 to 80 pounds per bag depending on your lifting abilities.

There will be a lot of garbage to dispose of after demolition of a plaster wall.  If you are in the middle of a remodel job you likely have a dumpster outside that you can throw the plaster into.  If you are doing work before remodel job starts then you can also store the plaster in your garage until you have a dumpster in place. 

Most garbage collectors will not accept big bags of plaster so don't try setting them on the curb without checking with your garbage collectors first.  You will likley have a landfill somewhere in your area so check your local listings to find one close by.  You can call them to make sure they can take your plaster, but there shouldn't be a problem.  The one in my area charges 58 dollars per ton of debris.  Although that sounds like a lot of weight, the plaster adds up quickly so you may have to pay a bit for the disposal, but it's still cheaper than having someone else do it.  It probably isn't a good idea to try moving the debris in a car as it is too much weight.  Even in a pickup truck you will need to make several trips so you don't max out your weight limit on your truck.

Before removing plaster wall.

After removing plaster wall