Paint is by far the biggest difference IMO. Modern paints do not look anything like lacquer and to some extent enamel and it stands out like a sore thumb on modern builds. Second for me it the perfection that everything is built to these days that was not present in the past. It is subtle but I think it is something the eye picks up even if it doesn't register outside our subconscious. I always remember the article Harry Bradley did pointing out all the flaws in the Burgarin Merc that unless you really looked for them you probably wouldn't have noticed but your eye picks those types of anomalies up and it registers in the brain. And finally old pictures just look different than what we have become accustomed to looking at with modern digital pictures especially black and white pictures.