Everything old really is new again when it comes to airlines and the business traveler. So when carriers invent a new product—in this case, a category of service between costly international business class and the cramped confines of coach—you can be sure that there's nothing unique about the opportunities. Or the problems.

Delta Air Lines' announcement last month that it will join an expanding list of international carriers with a so-called premium economy class goes a long way toward solidifying the concept, which has grown unevenly for more than a decade. It's also a rerun of the maturation of business class three decades ago, when the growing price and service disparity between first class and coach required the creation of that cabin.

The problems with widespread adoption of a premium economy cabin? Same old, same old. But let's deal with those issues after you fully understand what the addition of a "fourth cabin" means to business travelers.

Faced with an unpleasant reality—airlines charge huge premiums for the comfort of a business-class seat and they offer cattle-car conditions in coach—Delta Air Lines is retrofitting its international fleet with its new Economy Comfort class. Beginning this summer, the nation's second-largest airline will have a class that it claims allows "relaxation and comfort to co-exist."

Economy Comfort class flyers will sit in a chair that offers about four additional inches of legroom and reclines about 50 percent more than Delta's standard international coach seat. That'll mean about 35 inches of "pitch" compared to the industry standard of 31 inches in coach. (In contrast, most business classes on international flights these days offer seats that turn into flat beds.) Economy Comfort customers will also get to board the plane before coach customers and get some additional in-flight perks.

The granular details remain sketchy, but Delta says that Economy Comfort will look and feel very much like Air France's Premium Voyageur service. No surprise, since Delta, Air France, KLM, and Alitalia are all in the SkyTeam Alliance together, and they often code-share on each other's flights. Delta's Economy Comfort announcement also comes at about the same that Continental Airlines has decided to install the Economy Plus service first pioneered by partner United Airlines. And all these moves come years after the U.S. airlines' international competitors started offering a split-the-difference solution between coach and business class.

If Delta isn't exactly breaking ground with Economy Comfort, and its offerings fall in the murky middle ground of premium economy services worldwide, the pricing does look friendly. It'll be a free upgrade for Delta's most elite members of the SkyMiles frequent-flyer program. Status-deprived customers will pay just $80 to $160 one-way more than coach. Lower-level elite members of the SkyMiles programs will get discounts off the upgrade price.

Do business travelers want a fourth class of service on international flights? Absolutely. British Airways, which launched its World Traveller Plus cabin in 2000, is rolling out an upgraded version this year. Asian airlines have begun adding their own versions of premium economy too. Qantas of Australia, which operates some of the longest flights of the world, has pushed its Premium Economy class to space levels that were restricted to business class just a decade or so ago. Some Premium Economy seats on Qantas' Airbus A380 super-jumbo jets offer as much as 42 inches of legroom. Chairs are 19.5 inches wide, two inches wider than standard coach seats, a real perk for today's jumbo-size flyers. And Air New Zealand has its own wrinkle: a SkyCouch, three coach chairs that convert to an integrated in-flight sofa bed.

The latest carrier to install a premium economy class is Alitalia, which now offers the Classica Plus cabin on its New York-Milan, Miami-Milan, and some New York-Rome flights. The service is certainly priced right: A walk-up business class flight to Italy runs about $9,000 roundtrip. That's three times the $3,000 price of a walk-up coach seat. But a Classica Plus seat, which comes with more legroom and recline, priority check-in and boarding, and a more liberal checked-baggage allowance, costs just $3,400 roundtrip on a walk-up basis. If you purchase your tickets in advance, Classica Plus seats can cost as little as $1,440 roundtrip.

"We're getting a nice mix of business travelers who want more comfort, but aren't allowed to book business class, and upscale leisure travelers who want a better experience than coach," says John Di Rienzo, Alitalia's New York marketing manager.

Di Rienzo also knows what's driving flyers into Classica Plus. "It's absolutely about the seat, the extra comfort," he explains.

So if premium economy is inevitable because it is what passengers want, why isn't this so-called fourth class more prevalent? After all, American Airlines and US Airways, two of the nation's five carriers with international service, don't offer it. Major international airlines such as Lufthansa and Cathay Pacific don't, either. Singapore Airlines tried a premium economy service as a replacement for traditional coach on its ultra-long-haul nonstops to Singapore, but dumped the cabin. And OpenSkies, the British Airways' boutique airline, repositioned its premier economy cabin as a second type of business class and decided to position itself as an all-business carrier.

Part of the resistance is one version of the travel industry's same old, same old: Corporate travel buyers, who dictate policy for millions of business flyers, have been slow to adopt rules about who can and who can't sit in a premium economy seat.

"You go in to talk to corporate travel buyers and they just don't have a box for premium economy," the top salesperson for one international carrier explains. "They have very nuanced rules for who can book first [class] and who can book business class, but they haven't even considered who in a corporation can book [premium economy] flights. So until the travel managers set policy, lots of business flyers don't know the cabins exist and don't know if they can book it."