THE ECONOMIC summit at Williamsburg provided a useful demonstration of allied common interest and civility. That's another way of saying that, to the chagrin of the attendant international press, it was something of a yawner. Having been criticized for letting their differences bloom extravagantly at the last such affair, the seven participating industrial democracies, especially the United States, seemed not only content but actually determined to make this one bland.

Margaret Thatcher left for home a day early to resume campaigning. But though the other six stayed on, they were in effect campaigning, too. Certainly the dominant sense of the proceedings was shared awareness of, if not always fully shared respect for, the domestic constraints on economic policy. President Reagan, being the patron of the most powerful national economy represented at the summit, did not do the one thing--make an unequivocal commitment to reducing American deficits--that his partners most wanted him to do. Presumably they knew beforehand that he could scarcely do for them at Williamsburg what he has resolutely refused to do for Congress at home. His twin themes of dedication to continued growth and vigilance against a renewal of inflation nonetheless carried the day. The Europeans do not think they will get the same relief from a recovery that Americans anticipate, but they are desperate to get what they can.

Mr. Reagan made this the first of the nine economic summits held since 1975 to make a major move on a controversial security issue. It may not have been the best forum in which to broach this sort of business, but not to have tried would have been to let an opportunity go by. With both Japan and France on board, the seven endorsed a statement giving Mr. Reagan the boost for new missile deployment in Europe that he was eager to receive, and a pledge of fidelity to arms control that he was eager to give. The Kremlin sought to deter such a statement by a missile-rattling threat of its own on the eve of the summit, but the seven heads of government held firm.

From all accounts, Mr. Reagan personally acted very much the leader of the alliance. He was "up" for the summit, he set its tone of constructive engagement with common concerns, and he mastered all the theatrical possibilities available in the splendid Williamsburg setting. The president will take political credit for his performance, and he has every right to, even though the world is not a substantially different place now that the players have gone back to their wracking cares at home.