When Moshe Arens left his job as the Israeli ambassador the other day and went home to replace Ariel Sharon as minister of defense, the hot topic was: what difference would it make?

The prevailing view was that the change would certainly buff some of the rougher edges in U.S.-Israeli relations, Arens being a smooth operator and Sharon being a roughneck with no reluctance to denounce U.S. policy out loud. Presumably, this could open the way for an easing of some of the Reagan administration's difficulties with the government of Prime Minister Menachem Begin. And that would figure, if we are talking about, say, the George Shultz difference at State-- except that Alexander Haig did not stay on in the Cabinet with heavy responsibility for U.S. foreign policy,

But Sharon is staying on. As a minister without portfolio; he will be a member of a Cabinet defense committee as well as the Israeli team conducting negotiations over Lebanon. Begin will still have the benefit (if that's the word) of Sharon's bull-in-a-china-shop approach to advancing Israeli security.

So we are talking not about Arens instead of Sharon, but Arens-plus- Sharon. And that can only mean positive reinforcement for those impulses of Israeli policy that the Reagan administration considers to be the most negative in their effect on U.S. interests and objectives: the Israeli rush to make peace with Lebanon; the heel-dragging on a complete Israeli withdrawal; the flat-out opposition to resumption of the Camp David peace process under terms of the "Reagan initiative."

By way of evidence, I would cite an advance copy of an interview with Arens in the forthcoming issue of Trialogue, the quarterly review of the Trilateral Commission. The interview was conducted before Sharon's resignation, and Arens comes across with more candor--and contention--than in his guarded farewell statements as defense minister-designate.

Item: the administration wants to ease the pressure on Lebanon's president, Amin Gemayal, for a quick peace treaty with Israel, preferring to talk merely of "normalization." Arens is scornful of Israel's deference on this point. "My own opinion was that (a peace treaty) should have been the first step and one on which we should not defer," he declares, rejecting the argument that a separate peace treaty with Israel now would damage Lebanon's relations with other Arab countries.

Item: the Reagan plan called not only for an end to further Israeli settlements on the West Bank during the five-year "full autonomy" transition period envisaged by Camp David but recommended "the immediate adoption of a settlement freeze by Israel" last September. Begin's answer was to announce new settlements. Arens' answer is that any freeze, ever, on settlements would imply that Jews and Arabs cannot "live together," in which case, "I do not think that we are going to have real peace in the area."

Item: U.S. diplomacy is currently directed at some sort of arrangement under which both Jordan's King Hussein and the PLO would be folded into the Camp David process, having made the requisite commitments to Israel's right to exist and to the fundamental principles of Camp David. Arens argues that, thanks to Israel's Lebanese invasion, the PLO has been "greatly weakened." But he puts no stock in the notion that this will encourage real accommodation between Jordan and the PLO.

"A year ago, the kind of ballet--or, is it not, rather, shadowboxing?--now going on between King Hussein and PLO chief Yasser Arafat would probably have ended up with one of them shooting the other," Arens says, "and I wouldn't be surprised at all if, even at this stage of the game, they each carry a dagger or a gun in their pocket."

His point is that "moderation" on the part of the PLO has to end in its "disappearance," leaving Hussein as the sole spokesman for the Palestinians. The stage would then be set, Arens argues, for King Hussein to follow the example of Anwar Sadat. This example, when you think of the ultimate consequences for Sadat, might not recommend itself to Hussein.

But Arens is in no hurry: "The next step is for either King Hussein or President Assad of Syria to come forward as Sadat did. . . . Until such time, nothing is going to happen." Arens is also philosophical. "The peace process in the area is essentially a slow one; I think it is unfortunately going to take a little time."

It is right here that Arens comes most directly into conflict with the Reagan administration's thinking--and that of the so-called moderate Arabs as well. They are talking in terms of a quick solution--before the American presidential political process overwhelms serious Middle East diplomacy.

I am not suggesting that Arens brings brand-new thinking to the Begin government as defense minister. What he brings is forceful advocacy of much more of the same