I wrote this in a forum thread started by a guy who wants a commuter bike and has a budget of $250 - not enough to buy any but the cheapest of new bikes, but enough to get quite a lot on the used market. I'm saving the part about shopping for an older road bike. My assumption is that the person shopping wants to get something inexpensive that they can start using right away or after a tune-up and maintain in pretty much the same configuration. Additions are in italics.
There are some things to watch out for on older road and touring bikes that can make them harder to maintain. 27" wheels were common in the early '80s, and while you can still get nice tires for them, it's harder and replacing the wheel or rim is also harder. Dropout spacing and rear cog styles also changed a lot - in the '70's, a five-speed freewheel was common and by the end of the '80's or at least the early '90's, an 8-speed cassette became common on higher-end bikes. There were eight-speed freewheels for a little while and a lot of people had problems with bent axles.
I would consider a road bike with 700c rims and an 8-speed cassette to be a pretty kickass buy because almost all the parts on it are in current production and easily replaced and the dropout spacing should match the current standard. I wouldn't worry too much about 27" rims, as long as they're alloy, or a 5-7 speed freewheel.
Unless the bike is steel, you're committed to keeping the same dropout spacing. That means you can't upgrade less than an 8-speed cassette to a modern drivetrain. Steel bikes can be respaced. Also watch out for stem shifters and "suicide" brake levers. Stem shifters are very awkward to use and point at your groin, so you may run into them climbing out of the saddle, crashing, or straddling your bike at an intersection. Suicide brake levers don't have enough mechanical advantage to apply much force at the brake and reduce the throw of the brake levers, which can make the brakes harder to tune so they work. Some 27" rims were made out of steel. Braking performance on a rainy day sucks - it takes a few revolutions to dry the rim enough for the brakes to work, and you may already have rear-ended a bus by the time that happens (didn't quite happen to me, but it was terrifying.) Some steel rims didn't have a hooked bead seat, preventing the use of high-pressure tires. But you shouldn't buy a bike with steel rims in the first place.
The compatibility and similarity in performance of 80's and early-90's bikes with today's gear is surprising. Without the rack and wire baskets, my cheap mid-80's aluminum commuter weighs less than my nice '99 steel road bike. But the resale value sucks, so you can pick them up for cheap and they're relatively unlikely to get stolen.