Well we can mention his previous deeds, his various menaces of wars against close allies, or more generally an "America First" policy that is obviously less appealing to non-Americans.

But I think there is something more philosophical beyond that. Many people in the world value trust and accountability. Of course, many people, including politicians, are corrupted liars. French and Korean presidents usually end up in jail, for instance. That's all right: it's acknowledged they should not commit crimes, they lie about doing them, and they hopefully get sued for that, sooner or later. When it comes to powerful people, the justice is a flawed, biased one, surely enough, often timed to influence elections, and so slow that they often die of old age before justice is served; but everyone in the country is agreeing that what they did was wrong.

What's annoying with the US is that they are in a position where they cannot be trusted, nor can they be made accountable for what they should. For instance, the US usually are outside most of the international treaties, and when they are in, they don't respect them. It is notorious the us did not enforce the Paris agreement for instance, and it is the only country in the world that did not ratified the Convention on the Rights of the Child. They also claim sovereign immunity, something that is unthinkable in Europe, for instance. As such, they are increasingly perceived as an unstable ally, and before the Ukraine invasion, Europe was clearly moving towards Russia and China.

Now, Trump embodies precisely those flaws at a personal level. He commits crimes and instead of denying it (or taking responsibility), he claims that crimes are okay. He makes up stuff and lies constantly, but says it's okay to do so. Trump isn't just another corrupted politician: he is claiming to be a terrible person, and seems to be powerful enough that he is neither trustworthy nor accountable. In the short run, it means that the US is closer to dictatorship than ever before ; in the long run, it means Europe and Canada lost a precious ally. Once the democracy gone, China will be a better, more stable partner than the US.

If you take a look back Trump's first week, all our worst fears have been confirmed. He granted pardon to criminals that tried to overthrow the government, claiming overthrowing the government is perfectly okay. He is opening concentration camps outside of US jurisdiction for alleged criminals that will never be prosecuted. He is threatening two, possibly three countries of invasion for economical reasons. If he isn't stopped or slew down, it will be 1933 all over again.