I would say that we have two choices here, based on how recognisable you want the world to be.
The first is for slavery in any form to be declared a crime against god by theologians in the late 15th-early 16th Centuries, and as a direct result several powerful and devout Christian monarchs (followed by less powerful ones who see where the wind's blowing) outlaw all christians in the realm from taking part in any way in the owning or trading of slaves.
While this still leaves some leeway for plantation slavery, it's definately going to be much smaller in scale or possibly non-existent if said monarchs also don't trust non-christians to run the plantations (likely). This probably completely alters the entire colonial history of the Americas and Africa however, so things are unlikely to be familiar.
The Second is what I shall term the 'triumph of world liberalism'. Starting in the late 19th Century reformers are more succesfull in Russia, Austria-Hungary, Germany and the Ottoman Empire. Bismarkian diplomacy continues to hold Europe in a tense peace until his death in the early 20th Century, and it takes at least another 20 years for an alliance system to come into play. Because by this time all powers are too strong to hope for a quick war, we instead have an extended cold war style situation rather than a World War. Western Governments take the attitude that 'the white man's burden is to establish proper government in colonial areas and the means whereby the natives can be correctly educated to take part in this government', which has the side effect of colonies getting better education and transportation funding. Colonies are considered to expensive to run and mostly unprofitable, so the attitudes are that they should either be given enough self-government that the central power doesn't have to take the burden of funding, or if promising made more profitable with a view to a lower level of self-government. This has the side effect of moving towards more of a British Dominion approach to independence rather than post-colonial civil wars and unstability.
Japan is treated as a slightly inferior but still important ally by Europe in their cold war and so feels included enough that the rampant militarism is toned down, while China continues to see mismanagement and foreign influence for a longer period of time, but avoids the Maoist regime's worse issues.
Anti-Semitism probably lasts for longer, but a lack of Balfour Declaration, more limited Zionist movement that moves for just cultural autonomy within Palestine due to lack of hope for indepenendence, and less need to unite the country by targeting an ethnic minority mean we avoid the Holocaust and Arab-Israeli wars and aftermath.
This is probably absurdly over-optimistic though.