When President Bush unveiled his own $1.16 trillion version of former President Reagan's 1990 budget Thursday night, it quickly became apparent that the differences are ones of emphasis, rather than any radical changes. Unfortunately, the Bush budget does little to grapple with the federal deficit.

It calls for reducing the fiscal 1990 deficit to about $91 billion, slightly less than Reagan's figure and well under the $100 billion mandated by the Gramm-Rudman law. But where the $70 billion will come from to slash this year's $160 billion deficit was conveniently vague.Bush did not ask for dramatic cuts in spending and even took back some suggested reductions in domestic programs outlined in the Reagan budget as part of his "kinder, gentler nation" theme.

While Bush's proposed spending priorities hold the line on the military and put more into social programs - items that may defuse some of the sharpest congressional criticism - it is clear that the "honeymoon" between Bush and the Democratic-controlled Congress is about to start stumbling over budget specifics and hard choices.

Bush proposed to a joint session of Congress that defense spending be "frozen" - allowing only enough growth to keep pace with the rate of inflation. This would save an estimated $2 billion to $6 billion, to be used for education, the environment, AIDS research, and fighting drugs. Bush also would keep the $5 billion that Reagan wanted cut from Medicare.

The new president's most controversial budget priority is a proposed reduction of the capital gains tax to 15 percent, instead of the current 28 percent level. Bush claims the cut would encourage investment and eventually result in $4 billion more revenue. Democrats dispute that figure, arguing reasonably that a tax cut can't be calculated as income. In any case, they say it would benefit only the rich.

The capital gains proposal faces a stiff fight in Congress. Bush promised to work with and negotiate with Congress instead of engaging in bipartisan conflict, a cheering thought but one that is likely to be difficult to keep as priorities clash.

Democratic leaders in Congress want more funds to go into social programs, but nobody has explained where the money would come from. It ought to be obvious that Congress can't spend more on domestic social programs and still cut the budget.

One savings offered by Bush would involve cancellation of federal subsidies for community mass transit systems - a suggestion made several times by Reagan but always rejected by Congress.

In sum, the Bush budget is essentially the Reagan budget, with some adjustments away from military spending. With all its faults, the Bush spending plan is still a reasonable beginning. When Congress eventually gets through with it, there will be many more changes.

The critical part of the whole budget lies outside of Bush and congressional fiscal priorities and hopes. That critical factor is the economy and the tax revenue it produces. If estimates are too optimistic - and many economists believe they are - then across-the-board budget cuts will be required by the Gramm-Rudman budget-balancing law, and all present spending plans will be scrapped.

Barring such an outcome, the Bush budget would allow most hard choices to be delayed for another year. But after that, reality ought to start setting in.