and what it tells you depends on how it's measured.
Back in the days of the IOR, an SA/D of 16 was considered normal. Roughly speaking, anything over that was penalized in the rating.
The way it was measured was 100% foretriangle (I x J) and 100% mainsail (P x E). Most boats sailed with 150% genoas in light to moderate breezes. If you measured them, that put your "working sail" SA/D nearer 18 or 19. (I did an article about all this for CW some years ago, and have all the numbers somewhere).
Fast forward a few years, and your average cruising boat from Huntebenelina brags an SA/D of 18 or 19 . . . but that's measuring the full area of the jib -- the normal working sail (with a jib now in the region of 125% or maybe less).
So . . a 1980-ish boat with an SA/D of 16 has the same "power-to-weight ratio" as a modern boat with an SA/D of 18-19 (if you're using the Mfr's numbers.
The there's the D/L ratio (displacement:length). Your modern boats have lower DLs mainly because they have longer waterlines. That's why they are faster than older boats ("Longer boat are coming to win us . . .").
Boats with fin keels and spade rudders are more agile than long keelers -- but the price is you can't walk away from the helm to take a pic of your terrier on the foredeck. And some cruising boats today, in my opinion, simply don't have enough keel area to balance the sail area in stronger winds. (I wrote an article about that for Practical Sailor.)
I imagine the quest for the best-fit boat is difficult, especially if you're looking for space and amenities in a shorter boat. Maybe there's a longer, older, well-mannered, and well-kept boat that will fit the bill. I hear someone on this board has one for sale . . .