That's not how radio travels though. Instead of thinking of it as a beam that may be diverted by these, think of it as a bright light (omnidirectional) at a distance.

If there are "things" in the way, some of that light will bounce off and cast a shadow behind those things. Or they may reflect some of the light against the neighbors and you'll get some illumination behind obstacles. Some of these obstacles can be transparent to the light, translucent, semi-opaque or completely opaque (I.E. Paper, Wood, Concrete, Concrete with rebar, Steel facade).

But since the signal travels in all directions, there won't be enough attenuation caused by these that are any more significant than actual obstacles to reception.

A bigger problem may be interference from neighboring devices that function the same.

Actually if you are close enough to the transmitter you can cause issues. I remember at school we went on a tour of the local transmitting station. It was used last century for worldwide transmission of BBC World Service. At the peak of operation in the 80s, it had ten 500 kW transmitters. Anyway, so apparently one of the local farmers decided he wanted to cut his electricity costs. I'm not sure exactly what he did, but it was something like this (on a bigger scale), which did actually cause detectable issues. I think he ended up in prison after they caught him :s

Observing the situation when I'm in the densely populated area where I have problem in communicating with my own WiFi base because of the noise from other people's stations, I can imagine exactly two potential problems:

1) In densely packed areas, the devices first installed stop working as soon as there are more devices, due to noise.

2) In far-away places, not enough energy for devices to work.

It's not noise from other WiFi devices that cause problems in denay packed environment. The problem is the available bandwidth is consumed by other devices. Bluetooth uses the same frequencies as WiFi, but overcomes the problem by using frequency hopping and a method of adapting to the environment.

Does the increase in radio receivers require an increase in the energy to broadcast to the same area?

Nope. If it were inductive or capacitive transmission, this would be the case, but for far-field radiation, it's generally not.

It's more Shannon-Hartley: http://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theorem (I think).