It'd be easier to ask what didn't Buckley do. Basically all that which ailed American conservatism and neutered its effectiveness at withstanding & rolling back left-wing gains between Eisenhower and Trump can be traced back to him, either as an active proponent or at minimum as a figurehead.

Buckley set himself up as the gatekeeper of American conservatism through his ownership of the National Review & influence on other conservative organizations such as Young Americans for Freedom, a position which he used to lock out & demonize proto-paleocon groups like the John Birch Society as I have said. On the other hand, he got the neocon infestation going by inviting 'ex'-Trotskyists like James Burnham (a man whose Machiavellian idea about the emergence of a dominant managerial elite inspired the character of O'Brien in 1984, all Orwell was missing was Burnham's suggestion that such an elite continue to keep some democratic trappings like a token opposition and a 'controlled circulation of the élites' to better manage dissent...y'know, like what the uniparty has been doing for ages) to work for the National Review.

The creep of lolbertarian thought into the right, resulting in the joint promotion of social atomization, corporate worship and the capitulation of the levers of state power to the left, all with the inevitable consequence of the sixty-year losing streak in the culture war? Again Buckley's work, the 'fusion' he was responsible for in regards to the 'fusionist' trend which defined post-1950s American conservatism refers to him fusing conservatism with libertarianism and onboarding yet more 'ex'-Communists (this time supposedly libertarians) like Frank Meyer onto his platforms.

Israel worship to the detriment of any points of relevance to the American situation, or really anything else? Yet again, blame Buckley. He was a huge, huge philosemite, and the John Birch Society being dogged by accusations of anti-Semitism (even though they accepted sufficiently, consistently hard-right Jews into their ranks) because they dared note the connection between neo-Marxism & the huge number of Jews pushing its various strands (queer theory, the commie infestation of the civil rights movement, etc.) and because they supported a generally non-interventionist position in foreign affairs outside of rolling back Communism (meaning no dying for Israel) was what drove a rift between him & their leader Robert Welch, even before their final split. Buckley would also condemn Pat Buchanan on similar grounds.

Conservatism being liberalism driving the speed limit, essentially the manager of decline for the right's 'managed decline'? Buckley's own career and 'evolving' beliefs is an example of that. He went from opposing the civil rights movement to not only supporting desegregation, not only supporting integration, but also supporting affirmative action - his best friend & brother-in-law L. Brent Bozell meanwhile maintained a more consistent record, opposing him opposing desegregation but not drifting way off to the left (actually, he became a tradcath integralist willing to throw hands to combat abortionists, at which point Buckley thought he had gone the same way as the JBS). He was for wars abroad until they became unpopular, as was the case with Iraq. He quailed before gay critics and promoted the beginning stages of the slippery slope to today's hellscape. And so on, so forth.

Nine months after his death, his own son Christopher not only voted for Obama, but felt the need to publicly announce he did so in the pages of the National Review. Conservatives called him a traitor, of course. But in hindsight, considering all the 'contributions' Buckley had made to American conservatism, I don't believe it can be said that he betrayed his father at all (certainly Buckley himself thought he'd live to see the first black president and believed that was something to welcome) - he was merely carrying out the logical endpoint of his father's life's work.