Where: Looking west along Queen's Road Central. In the distance there's a gable with a circle:
That building was on the corner of Jubilee Street, just past the Central Market.
How about the buildings in the foreground, where were those? The first building on the right pins it down:
"The Eastern Bazaar, Leading Silk Mart, 35 Queen's Road, Central". Comparing maps from 1911  and today, number 35 hasn't moved. It's where Commercial House stands now, on the southeast side of Chiu Lung Street.
When: The back of the card suggests the 1920s:
See the stamp-box, which says "4 Cent Stamp Required"? I've got other postcards which were posted with 4-cent stamps. Their postmarks are dated in the 1920s. (If we have any stamp collectors reading, I'm interested to know which years the 4-cent stamp was used for postcards.)
The building left of the Eastern Bazaar helps us narrow down the dates some more:
Madame Flint was a well-known milliner and dress-maker in the early 20th century. She was successful in those businesses, but things fell apart when she formed a partnership with a Monsieur J. L. Durand. Together they expanded into other lines of business, lost money, and ended in bankruptcy. Her misfortune is our benefit, as the bankruptcy, and the events leading up to it, were reported in the newspapers :
[...] In May 1924 a certain partnership arrangement was made between [M. Durand] and Madame Flint. [...]
[There follows an examination of M. Durand by the Official Receiver, Mr E. L. Agassiz]: When you first went into partnership with Madame Flint, where was her business being carried on? In Queen's Road.
You subsequently moved into China Building? - Yes, on the first floor.
You then moved again to No. 12 Pedder Buildings - Yes, and that cost us about $20,000.
The Au Chic dress making business in Pedder Street, from January 1926 until January 1927. shows a profit of $3,149? - Yes. For the conditions in Hong Kong it is quite a success.
They were already in Pedder Building by Jan 1926 at the latest. I guess they moved into the China Building in Jan 1925, so 1924 was the last year Madame Flint's shop was on Queen's Road, and the latest date this photo could have been taken.
I'll date the photo to 1924, but if you can spot any other clues for dates, please leave a comment below.
Who: There aren't any western faces in the crowd here, despite the presence of shops like Madame Flint's. We're on the border between the European and Chinese sections of town - the nearby shop signs are a mix of English & Chinese, but looking further west they change to all Chinese.
What: Manual labour. No horse-drawn carts or motor-vehicles to be seen, people are either walking, pulling rickshaws, or carrying loads using shoulder poles. Were Hong Kong's roads really this clear of motor traffic, or had the photographer waited for a moment when the view looked good for a postcard?
And before we leave this photo, note our old friend lower-left: a rat bin! 
- Mapping Hong Kong, Plate 3-4
- The Hong Kong Daily Press, 1 Nov 1927, page 4
- Rat bins