For John W. Sewell, president of the Overseas Development Council, the Cancun summit meeting was moderately disappointing. ''It was a missed opportunity,'' said the head of this independent, nonprofit organization founded to increase American understanding of the economic and social problems of developing countries and of the importance of this third world to the United States.
Mr. Sewell believes the industrial nations should take urgent action to help the developing countries relieve grinding poverty. The Reagan administration, in his view, is more satisfied with the current situation; thus little or nothing of a concrete nature was accomplished at the summit.
Perhaps the main achievement in Cancun was hardly noticed. Four OPEC ministers, including Sheikh Ahmad Zaki Yamani of Saudi Arabia, got together there and, according to one report, paved the way for a unified oil price for the cartel.
Carlos Miravalles writes in the Journal of Commerce: ''Observers of the world oil situation in Caracas said informal decisions reached by the OPEC ministers in Cancun ultimately may prove more important than all the lofty rhetoric concerning eradication of hunger and poverty.''
The article speculates that the OPEC ministers have agreed on a new benchmark price of $34 a barrel. This would involve Saudi Arabia's raising its price $2 a barrel and most other OPEC members reducing their prices. This deal would be sealed at the emergency meeting OPEC has scheduled Thursday in Geneva.
One crucial factor, according to this Caracas report, is the US Senate vote on the AWACS deal tomorrow (Oct. 28). If the Senate vetoes the deal, the Saudis will apparently be more willing to raise their price and cut production by perhaps 1 million barrels a day.
As for the summit itself, it did not reach solid agreement on:
* How to continue the ''North-South dialogue'' between the rich and poor countries. The United States approved the continuation of talks within the United Nations, where the poor countries have a solid majority. But there was no decision on where decisions will be made. The US apparently still expects any UN proposals to require ratification in such agencies as the International Monetary Fund, the World Bank, or the General Agreement on Tariffs and Trade.
Such UN talks have been going on for years without major accomplishments, causing great frustration and boredom among the participating diplomats. The same indecision could last years longer unless the industrial nations give way to more of the poor-country demands.
* Setting up an energy affiliate of the World Bank. The idea was that the OPEC countries could give large amounts of funds to such an affiliate without upsetting the voting structure within the bank itself. The industrial countries do not want to give up their control of the bank, which might result from a direct capital contribution by the OPEC nations with oil money surpluses.
Proponents of the affiliate argue that it could use the World Bank's bureaucracy, thus not creating a new international bureaucracy that the US would consider wasteful. And the energy affiliate would meet the needs of the poor countries for large sums to expand their production of energy. It is reckoned that the oil needs of the developing nations will swell from 11 million barrels a day at present to 24 million barrels by the year 2000. Oil demand in the industrial nations is expected to decline 2 million barrels a day to 64 million barrels.
Any such affiliate would require some US contribution, however, and the Reagan administration does not want to ask Congress for any new ''foreign aid.'' Moreover, it has great faith in the ability of the international oil companies to help the poor countries develop their oil and other energy sources if given a welcome.
The Overseas Development Council's Mr. Sewell does see one positive note in the ''education'' of the leaders involved. ''The long-run effect may be quite beneficial,'' he says. But it may be months or years before this education produces concrete developments.
Mahbub ul Haq, World Bank director of policy planning, would have liked to see the summit leaders agree on ways to expand aid to low- and middle-income developing nations. Concessional assistance could be reallocated to the poor countries; regular World Bank loans could be stepped up to middle-income countries. This would not necessarily come out of the government budgets of industrial countries. He would have liked to see agreement to hold such summits each year, and to set up compromise ad hoc forums to negotiate specific policy issues.
As it is, he's afraid the North-South negotiations will remain mere ''shadowboxing.''