After skimming through most of the posts in this thread, I thought I should make a few comments:

1. You should paint "sizing" onto the walls before putting up wallpaper. The sizing makes the wallpaper easier to remove when you want to take it down.

2. Never paint over wallpaper. It just makes it harder to remove.

3. If you find that you tear the surface paper off of drywall when removing wallpaper, then as long as the gypsum core is still intact, you can repair that damage without replacing the drywall:

a. Simply apply strips of fiberglass mesh drywall joint tape over the bare gypsum core and run it up on the paper on both sides of the exposed gypsum a good few inches.

b. Dilute white wood glue with sufficient water to make it into a paintable consistancy, and paint that over the fiberglass mesh tape. As the glue dries, it will bond the fiberglass to the gypsum core.

c. apply two coats of fiberglass mesh tape, with the strips of the second coat running perpendicular to the strips of the first coat. Paint with dilute white wood glue after each coat and allow time for the glue to dry.

d. Now skim coat over the fiberglas mesh with drywall joint compound. There are two good ways for a total newbie to do a decent job skim coating:

e1) Spread the joint compound on the fiberglass mesh with a "V" notched adhesive trowel. Then, mist the surface of that joint compound with a spray bottle and trowel the ridges down flat with the unnotched edge of the trowel.

or,

e2) Spread the joint compound on the fiberglass mesh with a "V" notched adhesive trowel, allow the joint compound to dry and lightly sand the surface to knock off any blobs that are sticking out proud of the trowel ridges. Then, fill in the trowel ridges with more drywall joint compound using the unnotched edge of the trowel.

4. The reason why drywall is fairly strong and rigid considering what it's made of, is because paper is very strong in tension. Try pulling a $1000 bill into two pieces by just pulling real hard on it and you'll see what I mean. In order for drywall to bend, then the paper on one side of the drywall or the other has to stretch. It's the strength of paper in tension that gives drywall it's strength and rigidity.
"Reinforced concrete" is designed much the same way. Reinforced concrete is nothing more than normal concrete with steel rebar embedded in it. If the rebars are embedded in a concrete slab near the top of the slab and near the bottom, then for that slab to bend, the steel rebar on one side of the slab or the other has to stretch, and steel is very strong in tension.
Reinforced concrete and drywall share the same engineering design principles, and that's why both are stronger than they would be if the concrete and steel or gypsum and paper were assembled in an arbitrary manner.