The most obvious point, perhaps, is that Mary is using a linen cloth to hold her book -- an indication of how valuable books were before the printing press (by Gutenberg, c. 1440). The books are almost more important in the composition than Mary! The patron who commissioned the painting wanted everyone to see that he owned two books, a manuscript and a silk pouch to keep them in. High status indeed! It is possible that Mary is a portrait of his wife and even that Gabriel is the patron. If so, they are a wealthy young couple.
The gothic bench looks brutally uncomfortable, which is why, presumably, Mary prefers sitting on the floor. It has great gothic carving, and we note that the flower and leaf spandrels on the end appear almost unchanged in the corners of many 16th- and 17th-century English panels. The mortise and tenon joints between stiles and rails are double pegged, but those between muntins and rails, which have less work to do, are held with a single peg. Some English joiners 250 years later used the same technique. The wood is unstained but has probably been finished with a clear wax, like most 16th-century English furniture (17th-century patrons appeared to prefer their oak stained dark.)
There is also a foot board, presumably to keep feet off the stone floor in winter when it would have been cold. Unlike English floors, the stones are not strewn with hay or rushes, though they may have been in winter.
The shoe-foot table is interesting: one side of the support is well shaped, the other is plain. We wonder if the table tilted, so that the top rested against the straight side.
The window combines the three types of window common in the 15th and 16th centuries: the upper section has small leaded panes (very expensive); the center has an opening that can be closed with shutters (a choice between light and the weather of the moment); and the lower section is covered with a wooden lattice to keep the worst of the wind out: the lattice may be lined with oiled paper or linen, when it would be weather proof, but would admit far less light than glass. The right-hand shutter appears to be able to cover the whole of the window, which makes us wonder why the left-hand shutter was necessary at all.
The candlestick (just extinguished, still smoking), the stoneware jug (used as a vase -- quite a modern touch) and the lavabo (for washing hands) are all available to today's collectors. Look how modern the towel is, and what a wonderful pivoting, gothic towel rail it hangs on!