Tax experts across the political spectrum were almost unanimous yesterday in calling President Reagan's tax-overhaul plan a major improvement over the present Internal Revenue Code, but they said it was far less than its advance billing as a sweeping reform or "Second American Revolution."

They praised as "brave" his proposal to curb lucrative write-offs for capital investment, which have wiped out the tax bills of many corporations and wealthy individuals. Most applauded his plan to abolish deductions for state and local income taxes, which favor residents of the Northeast and Midwest and upper-income taxpayers.

They hailed his proposal to lower tax rates dramatically for individuals and corporations.

But with almost the same uniformity, these former tax-policy makers from the Nixon, Ford, Carter and Reagan administrations said the president had missed what several called a "great opportunity" to fundamentally alter the way the tax system shapes American life.

The Reagan proposal leaves intact certain lucrative tax advantages for oil and gas drilling, insurance, charities, investments in stocks and bonds and tax shelters. The Treasury Department had proposed to eliminate these and other breaks in a sweeping blueprint released last November, but the White House decided to restore them after heavy lobbying from myriad interest groups.

"The November proposal would have been a landmark in U.S. economic history and it would indeed have been President Reagan's 'Second American Revolution,' " said Henry Aaron, a senior fellow at the Brookings Institution and co-author of a recent book on tax reform. "This proposal has a very different character. It promises significant improvements, not a revolution.

"All in all, I think we would have enjoyed the movie if we hadn't read the book," Aaron said.

Greg Ballentine, a former Treasury Department official under Reagan, also called the president's plan a "significant improvement" -- but from a perspective very different from Aaron's.

Ballentine describes himself as a conservative interested in lower tax rates; Aaron focuses more on the elimination of tax advantages for special groups of taxpayers and corporations.

"My God, if you hadn't heard of the November proposal and suddenly the president had said we'll have top rates of 35 percent for individuals and 33 percent for corporations, we would all have said: This is tremendous," said Ballentine, a corporate tax-policy consultant. "Call it change, call it overhaul, call it what you will, but it is truly dramatic."

But the former officials said Reagan's plan, once reviewed by Congress, might be no more dramatic than the 1969 and 1976 tax laws, which were billed as major reforms. Those measures dramatically curbed tax shelters, increased taxes on capital gains and restricted numerous business deductions.

Most explained their enthusiasm less as a testament to Reagan's proposal than as a measure of their low regard for the present tax system, which Ballentine called "a relatively miserable standard" that distorts choices for both individuals and corporations.

Moreover, they said Reagan appeared to have forfeited his goal of "simplifying" the tax code by proposing to retain features that encourage tax shelters and favor certain industries and investments.

"Any attempt to sell this as simplification is disingenuous," said Frederic Hickman, assistant Treasury secretary for tax policy under President Richard M. Nixon. "It would greatly increase the tax practice of lawyers and accountants. By shifting the tax burden back to corporations, it creates more pressure to avoid taxes in that area, which is where most tax lawyers and accountants work."

Still, these former policy makers emphasized that Reagan's plan would remove some important incentives blamed for distorting choices.

By curbing depreciation write-offs and abolishing the investment tax credit, Reagan's plan would be less generous toward capital investment than was the tax code in the Carter adminstration, according to Emil Sunley, an economist who was President Jimmy Carter's deputy assistant Treasury secretary.

Sunley added that lower tax rates would make up for some if not all of the difference, but the overall effect would be to reduce the advantage of industries such as steel that invest heavily in machinery over those that don't -- such as the high-technology and service industries.

While some industries, like oil and gas, would keep certain advantages under the proposal, the timber industry and sellers of whole-life insurance would be stripped of tax subsidies that have become important to their profit margins for several decades.

The former officials said unanimously that Reagan's changes in the Treasury plan appeared to be driven less by tax-policy considerations than by a need to accommodate certain interest groups and to raise enough money to offset rate reduction.

As evidence, most cited his plan to tax only the first $10 a month of health insurance that employers now provide as a tax-free fringe benefit. Higher-income taxpayers would lose the least from this approach.

Attorney Don Lubick, Carter's assistant Treasury secretary for tax policy, also called Reagan unfair for proposing to abolish the deduction for state and local income taxes -- an advantage for the high-tax states of the Northeast and Midwest -- while retaining tax benefits for numerous industries and other regions.

"Reform is in the eye of the beholder," said Princeton economics Prof. David Bradford, a Treasury official under President Gerald R. Ford. "The bottom line is, will we have a simpler, better system? The answer is probably. Whether it will be worth all the trouble, I wouldn't be so sure."

"If the president's plan were enacted, it would be a tax change as major as the 1969 act," Sunley said. "Would you call it reform? That's a tricky question. Congress will call it reform."