NYSE in Europe?

One obvious way the merger will make it easier for Europeans to invest abroad, Thain said, is that the combined market will allow them to buy U.S. shares denominated in euros. "If you have a euro bank account and a euro brokerage account, you can't buy things in dollars" without having to pay currency conversion fees, Thain said. The new arrangement "makes it much easier."

Andy Kershner, who runs a trading firm in Austin, predicted that once the two exchanges harmonize their technology, direct access to foreign markets for small investors via the Internet won't be far behind. "We're talking about democratization of the markets," he said. "We've gone from you have to be in New York to trade to electronic access for professionals to direct access [to U.S. markets] and now you'll be able to trade all over the world."

Some investors said the merger's potential impact is being exaggerated.

"I can't imagine any good reason why we should be any more invested in the United States as a result of this merger," said Lodewijk van der Kroft, director of institutional asset management at Theodoor Gilissen Bankiers in Amsterdam, which has about $10 billion in assets under management. "Perhaps the transactions costs will diminish to a certain degree, which will allow the thresholds to invest in the U.S. to be somewhat lower. But the transaction costs are already so low, and they are peanuts compared with the ultimate costs of being invested in the euro zone versus the United States or other foreign stocks."

University of Maryland business school professor Peter Morici argued that most of the changes associated with the merger would happen anyway. "With or without the merger, trading costs are headed south because of the growing advantages of electronic trading, the deregulation of international capital flows and the competitive discipline imposed by the Internet," he said.

There's also the very real possibility that the merger won't go through.

Under the terms of the deal, neither side has to pay a financial penalty to walk away. Several top European policymakers, including French President Jacques Chirac and European Central Bank President Jean-Claude Trichet, have urged Euronext to keep its focus on Europe. Germany's Deutsche Boerse AG has made a rival bid for Euronext and could raise its offer.

While the U.S. reaction has been more positive, it's still possible that Congress or other policymakers could react badly to the prospect of a merger that could make a Frenchman (Euronext chief executive Jean-Francois Theodore) second-in-command and possibly the future head of the New York Stock Exchange.

"This is the sort of thing where powerful economic forces within each country say, 'Wait a minute. Does this mean foreigners are running our stock market?' " said James J. Angel, an associate finance professor at Georgetown University.