The most distinctive aspect of last week's unsuccessful U.S. Soviet discussions on curbing strategic nuclear weapons was, in retrospect, how public the exchanges became. The American proposals and the Soviet rejection of them have been fully aired - a major departure from the way SALT bargaining went on in the Kissinger era.

For the super-secretive Kremlin to reveal, as Foreign Minister Andrei Gromyko did at his unusual televised press conference last Thursday, the detailed attitude of the Soviet side, including figures that the United States had declined to disclose, is nothing short of remarkable.

The intriguing question now is what bearing all that openess had and will have on the future of the negotiations. Only time will provide the authoritative answer, of course, but informed speculation seems worthwhile.

Was the hullabaloo (a word Russians are found of using) merely a way of staking out ground for later compromises? Were the Soviets, in fact, just posturing? Or did they really mean what they said? Are they prepared to see the SALT negotiations fail?

In descending order of importance, here are what Soviet sources and Western analysis regard as the main reasons for the flat Soviet rejection of both American propositions - a broad plan for "deep cuts" in weaponry and a narrow formula for "capping" the race in strategic arms:

The so-called "comprehensive" U.S. plan would mean a significant cutback in present Soviet strategic strength and the Soviets are deeply suspicious of such a step. The Americans, they seem to believe, would derive benefits that the Soviets would not. This is the origin of the basic Soviet claim that the proposals are "inequitable" and "one-sided." Internal bureaucratic and political presents, in effect, an abrogation of the Vladivostok accord reached between Soviet leader Leonid Brezhney and then-President Ford in 1974. Having already gone through the complex internal bureaucratic and political peregrinations necessary for a major arms accord with the United States, the Soviets, are resentful of being told to start over.

The narrow U.S. option for ratifying Vladivostok, leaving aside the unresolved issues of the U.S. cruise missile and the Soviet Backfire Bomber, favors th e Americans, Moscow maintains. They feel the cruise missile is a much more formidable weapon than the Soviet bomber - or so the Kremlin contends.

President Carter's flambuoyant support of Soviet dissidents in the early weeks of his administration alarmed Soviet leaders, who saw th e American gestures as deliberately provocative and therefore a sign of ill-will. This greatly increased their skepticism about the U.S. plans. Brezhnev and his colleagues were disposed to be opposed.

There is also a purely American suggestion that the Soviets are immobilized because of the incipient or perhaps active succession struggle under way to replace the ailing Brezhnev. Verifying that claim is impossible for any outsider.

All these explanations for the Soviet rejection would have been equally possible had the negotiations been conducted in the atmosphere of confidentiality that previously prevailed, but Moscow might well have handled the matter less dramatically. President Carter's extensive public declarations of what Secretary of State Cyrus R. Vance would be offering put the Soviets in a position, they plainly felt, of having to justify their response.

Kremlin sensitivities were further exacerbated when Vance, and particularly Carter, gave an accounting of what had happened within hours of the events last Wednesday. The American versions understandably tended to portray the Soviets as turning down out-of hand a reasonable and well-intentioned U.S. proposal that would radically reduce nuclear arsenals.

The Soviet leaders see themselves, and more importantly want to be seen at home and abroad as champions of disarmament. So they evidently considered as similarly public and eye-catching forum as their only recourse to answering the Carter-Vance presentations.

Moreover, to explain why they had so firmly rejected the American proposals it was necessary to denounce them - which is what Gromyko did at the press conference.

At a briefing last Friday, White House National Security adviser Zbigniew Brzezinski said of the Soviet outburts, "There was just a tiny touch of defensiveness." That was a very substantial understatement.

Over they years, Gromyko had asserted in his often-heated remarks, the Soviet Union had put forth 70 proposals "that are meant to insure detente, peace, disarmament" - with the implication, since repeated in press commentaries, that Moscow cannot possibly be blamed, as the Carter administration is doing, for blocking a SALT pact.

Such Soviet defensiveness does not bode well for a mood of compromise between now and the Vance-Gromyko talks scheduled for May in Geneva. Maintenance of prestige is a matter of utmost importance to the Kremlin.

"In both cases," Alexander Bovin, a respected Soviet journalist wrote of the U.S. proposals in the government newspaper Izvestia Saturday, "the Americans were trying to build key policy on cunning and the desire to outwit their partner in the talks. And this is where it is not questions of military strategy that arise but questions of political morality and political psychology, questions of trust, sincerity, of elementary honesty without which any talks are deception and self-deception."