"Air travel is a commodity. The market still disciplines pricing," said John Heimlich, chief economist for Airlines for America, the industry's trade group. "Customers have the ability to cut back on the number of trips. And if I push the envelope too far with pricing, this other guy will be there in a couple of weeks."

What will happen to service?

Passengers probably have the greatest reason to worry about this. Combining airlines is complicated and difficult, and technological glitches are common. Employees of the airlines who are unhappy with the changes in their work conditions or fearful for their jobs can pose their own problems.

While American and US Air said the deal would mean better customer service, history shows complaints about problems such as lost bags, screwed up reservations, and delayed or canceled flights typically soar, especially right after reservation systems are combined some months after the deal closes.

"You cannot find an airline merger in recent times that went well," said Brancatelli. "The bigger the merger, the more problems there are. Computer integration is very complicated. If you records go awry, your bags will probably go awry."

And while the airlines are promising to keep all the hubs, Brancatelli said the US Air hubs in Phoenix and Philadelphia are probably at risk, since they're close to other larger American hubs such as Dallas and New York.

What's next for airline mergers?

After a decade that took 10 major airlines down to four, this deal is probably the last big combination, at least for a while.

The four big carriers that will exist after this deal -- the new American, United Continental, Delta Air Lines and Southwest -- are each big enough that to combine any two of them would probably prompt antitrust concerns that would block a combination.

There are restrictions on foreign ownership, making deals between carriers in different countries difficult. That's one reason why Delta is only able to buy 49 percent of Virgin Atlantic.