There are several things that kinda muddles the waters here. First off, you're comparing old-AND-worn, with new-AND-shiny. If you'd have compared-old-but-refurbished with new-and-shiny, some of the differences might have disappeared.
Next is that you're comparing different geometries, and bike setups. Bike designs have gotten more aggressive over the years, so an old road bike may be more similar with a modern tourer. And modern road bikes are creeping towards (old) track bike designs.
So....
There can certainly be a HUGE difference between different bikes, but how much of it that can be unanimously assigned to vintage is hard to tell. Your '82 Puch vs an '82 full-bred race bike straight out of Tour de France might have been an equal revelation.
Frame material as such is probably one of the least contributing factors.
But lets just say that at an equal level of engineering effort, an aluminium bike is likely to end up stiffer than its steel counterpart, due to the differences in material properties. Not that I think anyone would call a steel track bike floppy....
I don't feel qualified to compare old vs new in that detail, I don't get to ride enough different bikes. But rest assured that there are still turds out there. Modern materials, modern engineering is not a fool-proof vaccine against poor products.
You should be able to expect a lighter bike, and the brifters though. Apart from that, geometry and fit would be more influential than the vintage.
And the features that you like for a short ride, responsiveness etc, might not seem quite as nice under a long haul.
IMO, the big thing is the indexed, integrated brakes/shifters. They just make shifting easier on every level except maintenance, easier to reach, easier to perform. Ramped & pinned rings and sprockets are nice too.
Now, on the front, indexed isn't quite as much of an improvement. You still need to ease up on pedalling to get a smooth shift, so the added task of hand tuning the shifter position isn't really keeping you from putting the hammer down anyhow. But the easy access to the shifter is still a bonus.
I guess accessibility is the feature of brifters that I like the most. I'd probably have been quite content with friction, if there'd been a way of shifting both down&up through the whole range w/o having to take the hand off the bar
One more thing vs old/new is that old bikes are much more likely to have clearance for fenders and/or wider tires. And if you want any kind of utility value out ot the bike, this can be real important.
