I was weened on Wingmasters from the 1960's and early 1970's and agree they're of better quality than the Wingmasters of today and far better than the Express and SP models. For me it's not a functional thing, but rather a fit/finish thing. The wood on the older Wingmasters was a lot better. I liked the looks better when they had the white line spacers for the recoil pad and grip cap. The ribs on the older ones were fit better and the wood/metal fit was better at the receiver. The Express and SP models are made more toa price point and I get that so I don't knock them for the tupperware furniture.
Still own two vintage Wingmasters, one being 20ga which was my first repeating shotgun and the other being my dad's 12ga 870TB which i Inherited. In 1998 I purchased a new 870 SP that never gave me any issues in the 8-9 years that I had it. In 2011 I purchased a 12ga 870 Synthetic 7-Round for home defense and it runs just fine and has never given me any trouble. The newer "cheaper" guns aren't as slick and smooth out of the box as the older ones, but they're far from terrible and once you get a flat or two through them they start to break in and cycle more smoothly.
On the other hand, the 870 American Classic 20ga I purchased in 2014 is every bit as smooth and slick as my older Wingmasters, though it's not quite as "loose" as it hasn't had thousands of shells through it like they have. Wood is still a little more proud than I would like around the receiver, but that's really my only nit-pick on it.
