Are today's bikes "better" than a few decades ago? I'm looking to buy a road bike for commuting. There is a guy near me who finds older road bikes (e.g. from the 80's) and fixes them up. Are there downsides to getting an older bike (assuming it's had a proper tune up) compared to getting a bike released in the last year?
The last major technological improvement in standard bicycles was indexed shifting.  The indexing part of this isn't that big a deal, but the feature also gives you the ability to shift under load (which is a big deal).  I'm thinking this change occurred in the late 80s, but my memory for chronology is poor.

Yes, since then we've gotten V-brakes, carbon frames, "compact" cranksets and a few other things, but they're not fundamental changes, just minor refinements (if that).

Re "non-standard" bikes, the biggest change has likely been the improvements in geared hubs.  These have gone from 3-speed, don't-dare-shift-under-load-or-you'll-wreck-it models to multi-speed or continuous models that encourage you to shift under load.

There has also been progress in recumbent technology, but I don't think there've been any game-changers (though I'm not up on this).

The main problem with a bike more than 15-20 years old is that replacement parts get harder to find.  In particular, if you buy an older bike you might want to buy a spare rear cluster right off, to have on hand.  And I wouldn't recommend buying a road bike with 27" tires, since replacement tires (of the narrow "racing" variety) will be hard to find.