As someone who rides both, there is a difference, and a big part of it is simply that ergonomics are better now. Not just moving the shifting up to the brake levers, but also the design of those levers, the bars, the saddles, clipless pedals, etc.

More gears in wider ranges. For example, my 1980s bikes cam with 52/42 or 52/40 cranksets and 6 or 7 speed cassettes from 13-24 up to 13-32. My newest bike has a 52/36 crankset and 11-34 11 speed cassette.

Better brakes - my newest bike has disc brakes, but even my 90s and 00's bikes have dual pivot calipers that are much better than the older single pivot brakes from the 80s.

Better shifting - Hyperglide was introduced in about 1990, and it makes shifting much faster, quieter and more precise.

As far as fit goes, an old bike with a good fit is better than a new bike with a lousy fit, so there's that.