Thanks for all the interesting responses to my query.

I agree with Mistral's observation that it's the quality of the description not the quantity that counts -- though it doesn't answer the obvious next question of how to write the gem-like background description that in a few crisp sentences gives a diverse set of readers a feel for the setting/character.

Neil:

Description can also be used for authorial mouthpiecing, if you're into authorial mouthpiecing (which I am). I did this quite a bit in A Casting Of Swords ....

[examples snipped]

How anyone else actually reads it may be utterly different.

While I quite enjoy authorial mouthpiecing, if I'm recalling the right piece, then I thought the background description in this was a touch OTT -- I seem to recall skipping quite a bit while reading, and mentally substituting a cross between the seedier end of the docks in Portsmouth (where I was brought up) and those in Lisbon (pusher-laden).

Tavia