As someone once said, "Its a matter of what blows your skirt up."
SxS's were simply the venue for the gunmaker's art when it was formative and during its apex of both design & execution and as such can be appreciated on that basis alone .. or not.
I could argue that the SKB's of the not too distant past represent the penultimate modern machine made SxS wonder, extremely well fit and to an extent not always equaled by some better known and much more expensive names. It's well worth buying a clean SKB 100 and having it stocked to your liking, IMHO, but it is only that, an opinion. I doubt one can buy a better built gun for the money. OTOH, there are select guns from the same era or slightly earlier that had chopper lump bbl's, fine fit & finish & remain 'sleepers' in the present.
Most any SxS will be shallower in it's frame than an O/U of the same gauge. I honestly can't even think of a single exception.
As far as your question about the 'what is it' about a SxS vs. 'others'. It has twice the width looking down the bbls., for starters, perhaps aiding in seeing the muzzle in a peripherial sense against some backgrounds & it has some substance in your hand, but most importantly most of the time on targets of opportunity when rough shooting or behind a dog [assuming it is reasonably 'fit'] they come up natrually .. some say they are fast handling, but that is hardly universal as Rocketman's MOI readings clearly prove.
Don Zutz in his writings said it was more a 'weight balanced between your hands' thing and tried to get their essense & appeal into words. He did as good a job as any & better than most. Gough Thomas is another author who offers a lot of insight into the mistique of double guns.
More than you asked, but your question needs a good vetting now & again and I'm glad you asked it.
