What is "polite" sound?

Most likely would be some British speakers "driven" by a French amp.

Polite by me is interpreted as opposite of forward...a laid back presentation. I find Verity and Vandersteen fit this description.

"Polite" sound is often what reviewers say when they can't define a specific defining characteristic of a component or speaker (i.e. descriptors like deep bass or scintillating highs, edgy, bright, harsh, sibilant or recessed don't seem to apply etc.)

In a high quality system this is usually indicative of a good design especially when the reviewer just thinks the sound is very natural and convincing. It is another way of saying that nothing stands out like a sore thumb about the design and therefore the system is likely to translate well too most recordings and genres. (Unfortunately this type of system may not sell well in a shop demo, however, there is a higher probability that you will not tire quickly of a system that is polite)

'Polite' sound is a description of a system with restricted dynamic range and restricted frequency range.

Tobias seems to have gotten it about right.Those who describe the sound of a system or component as polite are probably commenting on a lack of resolution which is primarily manifest in lack of dynamics and speed.I would disagree about there being any relationship to high-frequency extension,though.My speakers only extend to around 12,000hz,and while some would not care for the sound,I don't think they would ever be described as sounding polite.

Polite systems struggle with contrasts,imprinting a sameness to the music which might lead to disinterest and more frequent napping.
Low sensitivity speakers with nonexotic tweeters and medium resolution electronics is a sure fire route to polite audio.