This is my *NON AUTHORITATIVE* opinion, based on my research in this matter via past discussions about this topic on Pianoworld and other sources.
1. What does low tension mean? Are the strings under less tension as in strung less tightly? If so, why doesn't this affect the pitch?
Low tension means the tenor strings are pulled to less tightly than some other scale designs. In order to maintain proper pitch at less tension, thinner strings are used. Think about tuning a guitar string. You can raise the A string (5th string) to the same pitch as the E (6th string) simply by putting more tension on it. Because A string is thicker, more tension is needed to get to the same pitch as the thinner E string.
2. What are the advantages and disadvantages of a low tension scale design?
There are no apparent advantages or disadvantages of low tension scale. It is one of the many choices made by piano scale designer.
3. Which makes can be described as low tension, high tension, or somewhere in between?
Low Tension Scale Design is a marketing term invented by Steinway to differentiate their Boston line of pianos from other pianos made in Japan. Steinway designed the Boston line and contracted out the manufacturing to Kawai. Steinway themselves do not describe *ANY* of Steinway pianos as being "low tension". It then begs the question: is low tension scale deign really better, since it's not being used in the "best pianos in the world".
Early in the Boston piano history, this marketing term was pushed very hard by Steinway salespeople. It's not so emphasized these days, since it has become apparent to the world that the low tension scale really doesn't matter. However, some sales people still follow the old marketing material (like the website mentioned by another forum member) and push the "benefits" of low tension scale design.
As far as I can tell, *all* the supposed unique features of a Boston do not make a large difference. I personally find Boston pianos to sound similar to Kawais, but much more expensive. I can definitely buy a larger, better sounding Kawai at the same price as a Boston.
Pianos should be evaluated by the buyer's preference in tone, touch, and their brand preference. As "The Piano Book" tries to explain in detail, a list of features is not a good indicator of quality of a piano.